what if I am trying to open an .avi video format file and, though, I am receiving the message that "QuickTime Player can't open this file because a required codec is not available"?

Download and install VLC Player instead. Quicktime does not have the native ability to play DivX / XviD files which is the most common codec used with AVI files.

There are also other media players with the capabilty.

Check the Mac App Store. Some are free, some are not.

They all have various features you may or may not need.

Flip For Mac is a pretty basic (the free version) player

for avi and other Wndows based video formats.

Install Perian

This adds the codecs to QuickTime

I wouldn't recommend Perian because the developers are stopping support for the program. Plus when I used it the program had problems with certain avi files and the audio of the avi file. I would highly recommend VLC. I have never experienced any issues plus it's a more advanced video player.

What cbs20 mention VLC is the way to go

http://www.videolan.org/vlc/index.html

Well I would recommend it simply because it works for me, and always has.