In simple terms, the swap partition or swap file is used when your installed memory is full and instead of the computer crashing it will place some of its memory in the swap partition.
The swap partition is also used during hibernation. All of the information in memory will be written to the swap partition so the computer does not 'forget' what it was doing when the computer goes into hibernation.
The general rule for swap partition size is equal to the amount of installed memory. In your case 16GB. There must be enough space to write everything in memory during hibernation.
If you do not use hibernation you can decrease (or increase) your swap size. Remember that when the computer uses swap instead of installed memory it will be much slower because reading from a disk take much longer that memory.