Pushdown automaton

A pushdown automaton is like a robot that can read and understand instructions. It's given a set of instructions (like a list of things to do) and it follows those instructions one after another. It keeps track of what it is doing using something called a "stack". The stack is like a pile of things, and the automaton can add things to the top of the stack or take things off the top as it needs. When it finishes all the instructions, it's done its job.