The numbers in parentheses
) refer to different sections of "the manual". If you do
man man, it will tell you which numbers correspond to which section of "the manual". Each operating system may use its own layout for man page sections. Here is the section layout of my Ubuntu system.
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 eg /etc/passwd
7 Miscellaneous (including macro packages and conven‐
tions), e.g. man(7), groff(7)
8 System administration commands (usually only for root)
9 Kernel routines [Non standard]
If you do
man 1 printf, you get information about the shell utility
printf, but if you do
man 3 printf, you get information about the
printf function in the C runtime.
The numbers in brackets
] refer to external resources and should have locations or URLs described further in the article you're reading.