How do I get the man command?

I want to add to elken's answer that apparently, the 'man-db' packages doesn't cover all the manpages. For example, I needed documentation for the C stdio library (fopen, fgets, ...), and for that I had to install 'manpages-dev':

sudo apt install manpages-dev

Apparently there are also some other manpage collections (/different names for them on certain unix distros), see https://superuser.com/questions/40609/how-to-install-man-pages-for-c-standard-library-functions-in-ubuntu

apt update to update the local package lists followed by apt install man-db to install the actual package.