First Corinthians 12: 27-28 says this:
All of you together are Christ’s body, and each of you is a part of it. Here are some of the parts God has appointed for the church:
first are apostles,
second are prophets,
third are teachers,
then those who do miracles,
those who have the gift of healing,
those who can help others,
those who have the gift of leadership,
those who speak in unknown languages
What many people take from this is that apostles hold the most important position in the church, the top of the hierarchy. Prophets are second most important, teachers third etc. So to be an apostle is to be the most important person in leadership. Like this:
The problem with this view is that leadership in the New Testament is not positional, but functional. Another way to view it would be like building a house. First you need an architect, then someone to lay the slab, then someone to frame the house. Later would come plumbers and electricians etc. It's a question of function. The architect is essential to lay the plans for the house, and his work comes before the work of the electricians and plumbers, but without all these other functions, the house would not be built.
An apostle is one who is sent out (meaning of the Greek word apostello). He/she is the one who is usually the first person in a place or people group with the good news of the Kingdom.