"Sourcing" an external file means to incorporate the contents of that file into your script at that point of the command, and your script will act exactly as if the lines were written there directly.
It can be called with either the "source
" keyword, or more commonly with ".
", a single period.
So if you have a file called "defaults.txt" which contains:
...And you source it at the top of a script like this:
Then the variable values should echo the settings from the file.
Sourcing is covered in the BashGuide here:
I recommend checking out the whole thing when you have time.