Resident set size

Resident set size is like the number of toys you have out to play with right now. Imagine you have a big toy box with all your toys inside. But when you play with your toys, you take some of them out and keep them near you so you can easily play with them. That's what resident set size is: the number of toys you have out and nearby at the moment.

In computers, resident set size is how much memory (storage space) a program is currently using. Just like you take out toys to play with, a program takes out some of the memory it needs from the computer's memory pool, and keeps that memory with itself so it can easily use it whenever needed. This memory may include things like the program's code, the data it is currently working with or even some frequently used instructions that help make it work faster.

So, if a program has a high resident set size, it means it's using a lot of memory and probably is working on something complex. But if it has a low resident set size, it means it's not using much memory and may have finished its task or is waiting for more instructions.