The splwow64.exe process gets executed every time a 32-bit application is accessing one of your installed printers. You can kill this process if you are not actively printing something. It will simply restart the next time you want to print something, e.g., from Word (which is a 32-bit application).
WOW64 is better known as “Windows On Windows 64” layer which enables you to execute 32-bit applications on a 64-bit system.
32-bit applications cannot use 64-bit binaries (and therefore also printing drivers) and vice versa.
splwow64.exe is a 64-bit application, therefore able to speak with 64-bit printing drivers and “translates” the communication between 32-bit applications and 64-bit drivers.