What is swap and how large a swap partition should I create? [duplicate]
Swap is what is used when you run out of RAM.
RAM is fast. SWAP is SLOWWWWWW (being as its mechanical (hard disk) as opposed to ram (purely electronic)).
In the old days, you allocated double the amount of RAM you had to swap. These days its not such a hard and fast rule. 2-4gb is the usual.
Related question: I have 16GB RAM. Do I need 32GB swap?