Simple Definition:

Apostle is a term used in Christianity [a religion based on the life and teachings of Jesus Christ] to refer to someone who is chosen by Jesus Christ to spread his teachings [ideas or beliefs that are taught or passed on] and establish his church [the group or organization of people who worship and practice Christianity together].

For example, the twelve apostles, including Peter, James, and John, were chosen by Jesus to spread his message and establish his church.

Very Simple Definition:

An apostle is someone chosen by Jesus to teach others about him and set up his church [the organization that teaches and practices Christianity].

For example, Jesus chose twelve apostles to tell people about him and start his church.