To put it very simply. SWAP memory is disk space that is being used as RAM. It's used when your server has no physical memory left.
So if you have a 10gb Server, and all 10 are being used, your server will start to use SWAP memory. SWAP is far slower than your physical memory, however, it can be useful.
Let's say you're using your server, and sometimes it peaks over your 10gb max, to 10.2gb. This would normally crash your server, however, with SWAP memory active, this will in most cases stop your server from crashing.
SWAP memory is active on most of our plans. If you would like more information about SWAP memory on your server, please contact our support