How does swap space work?

Can you please explain to me, in simple terms, how swap space functions in a computer system? I'm trying to grasp the concept of how it supplements RAM when memory is running low. Is it essentially a way for the computer to temporarily store data that can't be held in RAM onto a hard drive or SSD? And how does the process of swapping data back and forth between RAM and swap space affect system performance? Additionally, is there a recommended amount of swap space that should be allocated for optimal system functioning?