What is an apostle?
An apostle, as defined in the original Greek language, is a messenger: one to send a message or one sent on a mission.
By definition an apostle is one who goes. In the general sense, like a missionary. But there are major differences. The call of an apostle is to go to where the gospel has not yet gone, to where there is no current church and plant a new church in the community. Then pass authority over to pastors and elders by appointing them in every church and imparting spiritual gifts wherever he goes. Then the apostle moves on to the next mission leaving behind a fully functioning and healthy church. And so the apostle goes from place to place to where they have not yet heard the gospel, planting churches wherever he may go and revisiting them to help them become a healthy church.
What an apostle is not:
True apostles depend on God and support themselves where necessary, and will not ask the church to support them or their ministry.
2 Corinthians 12:14,15
Now for the third time I am ready to come to you. And I will not be burdensome to you; for I do not seek yours, but you. For the children ought not to lay up for the parents, but the parents for the children. And I will very gladly spend and be spent for your souls; though the more abundantly I love you, the less I am loved.