The register keyword is a request to the compiler that the specified variable is to be stored in a register of the processor instead of memory as a way to gain speed, mostly because it will be heavily used. The compiler may ignore the request.
The keyword fell out of common use when compilers became better at most code optimizations than humans. Any valid program that uses the keyword will be semantically identical to one without it, unless they appear in a stringized macro (or similar context), where it can be useful to ensure that improper usage of the macro will cause a compile-time error. This keywords relates closely to
register int x=99;