A common representation of an automaton in computer science. This automaton "accepts" all the sequences of the letters a and b which start with an a and end with a b.
An Automaton (one automaton, several automata) is a concept from mathematics. Sometimes the concept is called state machine. It is like an abstract machine.
Such a machine can be given input, which is either rejected, or accepted. It's like a vending machine. When something is bought, coins (or money) needs to be inserted into the machine. If these are the right coins, they are accepted, and the requested item is dropped so it can be removed. If the coins are wrong, they are rejected.