Swap usage refers to the amount of virtual memory that is being utilized on a computer system. Virtual memory is a combination of physical memory (RAM) and disk space that is used to store data that cannot fit into physical memory. When physical memory is full, the operating system will move data to disk space in order to make room for new data. This process is known as swapping.
Checking swap usage is important because it can help you to identify potential performance problems. If swap usage is high, it means that the computer is running out of physical memory and is having to use disk space to store data. This can slow down the computer because disk space is much slower than physical memory.