Why does the group of some file show number (looks like ID), not an alphabet string?

Normally as 'ls -al', the files always show like:

-rw-r--r-- 1 owner_name group_name size time file_name

But some of my files show:

-rw-r--r-- 1 owner_name 'number' size time file_name

And the 'number' does not exist in /etc/group

What does the 'number' mean? and how to fix it?

Many thanks!