What is swap and what is it for?

SWAP is a special disk partition or file to which the operating system moves individual blocks of RAM when there is not enough RAM to run applications.
 
You can read more about the swapping mechanism on Wikipedia.
 
In simpler terms, swap is "ballast" for RAM.
 
But it should be noted that since swap is placed on disk, its speed is much slower than RAM. Therefore, its use is not recommended.