Some of the programs benefit significantly when you use all the threads your computer’s CPU has to offer to your operating system.
The number of threads available can be larger than the number of physical (hardware) cores in the CPU (also known as Simultaneous multithreading).
For example, in Intel’s CPUs (those that implement its Hyper-threading technology) the number of threads is usually double the number of physical cores in your CPU.
On a GNU/Linux system, the number of threads available can be found with the command
$ nproc command (part of GNU Coreutils).
Gnuastro’s programs can find the number of threads available to your system internally at run-time (when you execute the program). However, if a value is given to the --numthreads option, the given number will be used, see Operating mode options and Configuration files for ways to use this option. Thus --numthreads is the only common option in Gnuastro’s programs with a value that does not have to be specified anywhere on the command-line or in the configuration files.