Working set size is like the number of toys you can play with at the same time without getting overwhelmed. Just like how you can only play with so many toys at once before it becomes too much, your computer can only handle a certain number of tasks at once before it gets bogged down and slows down.
The working set size is the amount of memory that a program needs to perform its tasks efficiently. Think of memory as a big toy box. Programs need to take some toys out of the box and use them for a little while, and then put them back in when they are done. If a program tries to take out too many toys at once, the box will become overcrowded and the program will slow down or even crash.
So, the working set size is like a limit on how many toys a program can take out of the memory toy box at one time, to make sure it doesn't get too crowded and slow down. This limit is important to make sure that your computer runs smoothly and efficiently, just like how you need to clean up your toys and keep your play area organized to have a fun and successful playtime.