To answer this question,
we should turn to Pope Francis, who introduced the term in his exhortation,
The Joy of the Gospel

We are All Missionary Disciples
In virtue of their baptism, all the members of the People of God have become missionary disciples (Mt 28:19)
In virtue of their baptism, all the members of the People of God have become missionary disciples (Mt 28:19). All the baptized, whatever their position in the Church or their level of instruction in the faith, are agents of evangelization, and it would be insufficient to envisage a plan of evangelization to be carried out by professionals while the rest of the faithful would simply be passive recipients. The new evangelization calls for personal involvement on the part of each of the baptized.
Every Christian is challenged, here and now, to be actively engaged in evangelization; indeed, anyone who has truly experienced God’s saving love does not need much time or lengthy training to go out and proclaim that love.
Every Christian is a missionary to the extent that he or she has encountered the love of God in Christ Jesus: we no longer say that we are “disciples” and “missionaries,” but rather that we are always “missionary disciples.”
Every Christian is a missionary to the extent that he or she has encountered the love of God in Christ Jesus: we no longer say that we are “disciples” and “missionaries,” but rather that we are always “missionary disciples.” If we are not convinced, let us look at those first disciples, who, immediately after encountering the gaze of Jesus, went forth to proclaim Him joyfully: “We have found the Messiah!” (John 1:41). The Samaritan woman became a missionary immediately after speaking with Jesus and many Samaritans come to believe in Him “because of the woman’s testimony” (John 4:39). So too, Saint Paul, after his encounter with Jesus Christ, “immediately proclaimed Jesus” (Acts 9:20).
So what are we waiting for?
Got a question? Let us know.

The Church is the “Missionary Disciple”
None of us walk alone: missionary disciples always live out their discipleship in the sacramental and missionary community of the Church which “goes forth” …
The Church which “goes forth” is a community of missionary disciples who take the first step, who are involved and supportive, who bear fruit and rejoice.
The Church which “goes forth” is a community of missionary disciples who take the first step, who are involved and supportive, who bear fruit and rejoice. An evangelizing community knows that the Lord has taken the initiative, He has loved us first, and therefore we can move forward, boldly take the initiative, go out to others, seek those who have fallen away, stand at the crossroads and welcome the outcast. Such a community has an endless desire to show mercy, the fruit of its own experience of the power of the Father’s infinite mercy.
Let us try a little harder to take the first step and to become involved. Jesus washed the feet of his disciples. The Lord gets involved and He involves His own, as He kneels to wash their feet. He tells His disciples: “You will be blessed if you do this” (John 13:17). An evangelizing community gets involved by word and deed in people’s daily lives; it bridges distances, it is willing to abase itself if necessary, and it embraces human life, touching the suffering flesh of Christ in others. Evangelizers thus take on the “smell of the sheep” and the sheep are willing to hear their voice. An evangelizing community is also supportive, standing by people at every step of the way, no matter how difficult or lengthy this may prove to be. It is familiar with patient expectation and apostolic endurance.
An evangelizing community is always concerned with fruit, because the Lord wants her to be fruitful.
Evangelization consists mostly of patience and disregard for constraints of time. Faithful to the Lord’s gift, it also bears fruit. An evangelizing community is always concerned with fruit, because the Lord wants her to be fruitful. It cares for the grain and does not grow impatient at the weeds. The sower, when he sees weeds sprouting among the grain does not grumble or overreact. He or she finds a way to let the Word take flesh in a particular situation and bear fruits of new life, however imperfect or incomplete these may appear. The disciple is ready to put his or her whole life on the line, even to accepting martyrdom, in bearing witness to Jesus Christ, yet the goal is not to make enemies but to see God’s Word accepted and its capacity for liberation and renewal revealed.
Finally, an evangelizing community is filled with joy; it knows how to rejoice always. It celebrates every small victory, every step forward in the work of evangelization. Evangelization with joy becomes beauty in the liturgy, as part of our daily concern to spread goodness. The Church evangelizes and is herself evangelized through the beauty of the liturgy, which is both a celebration of the task of evangelization and the source of her renewed self-giving.