Imagine that you want to compile and install a software from sources. You get the tarball, untar it, and run ./configure. The process fails because you don't have all the needed librairies installed. Normally you would find the packages installing those librairies, install them, and try the ./configure again. You will eventually have to reproduce this several times.
Debian provides a way to make this process easier:
-Install the auto-apt package with apt-get install auto-apt.
-Run auto-apt update, auto-apt updatedb and auto-apt update-local to populate its database.
-You can now run auto-apt run ./configure instead of ./configure, and the system will offer you to install the missing packages.
-You can also run auto-apt run command for any command that may require some missing files.