Every minute of every day of every week of every month, that command runs.
man 5 crontab has the documentation of this. If you just type man crontab, you get the documentation for the crontab command. What you want is section 5 of the manual pages which covers system configuration files including the /etc/crontab file. For future reference, the sections are described in man man:
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
6 Games
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]