The number refers to a section of the Unix manual. For example, grep is in section 1 (general commands), so the title of its man page is grep(1). Sometimes two different things have the same name, but they can be distinguished in writing by the sections of the manual they’re found in. For example, stat(1) is a command and stat(2) is a system call. To see a description of each section, man N intro, where N is a number between 1 and 9.