Nice explanation from Steven de Rooji
“Hi mom! Okay, this is it! First, sit down, take a deep breath. Have some tea. This is gonna hit you hard. Okay, here goes.
People have found out that some problems are much easier to solve with a computer than others. They have given names to groups of problems depending on how hard or easy they are. Makes sense? Okay.
The first group of problems is called P. Problems in P can be solved by a computer in a reasonable amount of time. Example: sorting a list of names is not so much work for a computer. You can remember it like this: “Easy PEE-sy!” right?”