What do the numbers in a man page mean?
All system calls described in manpages have an associated number such as
exec(3). What is the meaning of this number?
The number is the man section. The 3 is for library calls. Here is the full list from man(1):
Some things have the same name in different sections. For example, man(1) is for the man binary, while man(7) explains how the macros should be written for manpage formatting. To access a specific section, you can put the numerical argument before the item: