It's a way to send a SIGALRM signal to your running process [1] using the alarm() syscall. You can also catch the signal [2] and act upon. For example it's used by "timeout" [3].

[1] https://dokk.org/manpages/alarm.2#DESCRIPTION [2] https://dokk.org/manpages/sigaction.2#DESCRIPTION [3] https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/tim...