TonyLMiller,
Like Aay said, ...
apt uses a file, or list, or even a database, that it maintains, and uses, to hold all the "packages", or software that is available from the "sites" you have in the Sources.List file. Since you want to be sure that this list is as up-to-date as possible, so what you want to install, or look for, is the most recent version, or if it is a new program, it might not even be there, you want to "refresh" that database.
To refresh your database that apt-get uses, you run the apt-get update command - I tend to run it whenever I am searching for a program of interest, or needing to add a new program of interest, or, as you found out, whenever you changed the sources.list file.
Hope this helps,
Ms. Cuddles
