This is sometimes referred to as a significant comment. The intention is that of passing directives to the compiler or the preprocessor at various points in the source program. Such directives might indicate whether speed or space is more important, or if certain checks should be suppressed, or if a particular routine is to be timed, etc.
An instruction to the compiler to compile your program in a certain way. For example, you can use pragmas to insert copyright information into your object files, to specify a particular template instantiation, and to specify optimization levels.
a command that you can give to a particular compiler, but which will be ignored by other compilers