The first thing that you must accept is that profits come from wise investments. Building a bitcoin or crypto currency mine takes investment and is not a backyard gold mine.
The simplest answer to what it is:
It is a series of specialized processors that are trying to guess a very specific number called a Hash Rate. There is heavy competition, but when you get it right, you earn coins to your account. The more processors you have working at guessing the greater your chance to win.
This is the very bare bones explanation, but the more often you can guess the greater your chance to win. A mine is when you have several processors guessing. A successful or profitable mine is when they can make more than their overhead.