CodePudding user response：
Those are the manual section numbers. From the man manual:
The table below shows the section numbers of the manual followed
by the types of pages they contain.
1 Executable programs or shell commands
2 System calls (functions provided by the kernel)
3 Library calls (functions within program libraries)
4 Special files (usually found in /dev)
5 File formats and conventions, e.g. /etc/passwd
6 Games
7 Miscellaneous (including macro packages and conventions),
e.g. man(7), groff(7)
8 System administration commands (usually only for root)
9 Kernel routines [Non standard]
CodePudding user response：
The number refers to the section of the man-page.
Some (most?) topics have entries in multiple sections, so the section number is used to differentiate them.
Foe example, there's a topic for login in the User Commands section (i.e., login(1)) and in the Linux Programmer's Manual (i.e., login(3)).
