Swapfile is commonly refers to a file backed storage for paging. (aka swapping)
Modern operating systems can use disks (or other IO) to temporarily store in memory information. This allows them to write this information and reuse the main memory which contained it, with the ability to recover this information.
Any modern OS can do this with chunks of memory and it is called paging.
However historically only whole processes could be swapped out. Hence the traditional use of 'swap partition' or 'swap file'. In the case of a 'swap-file' the data is stored on a file, incurring extra filesystem overhead.