How can I watch .avi movies on my imac?

I am extremely basic user and I have a few .avi movies I want to watch but they wont open. It says 'Quicktime player cannot open .......avi."

Then "To see if additional software is available that will enable QuickTime Player to open the movie, click Tell Me More." But I don't know what I am looking for or what to do, can anyone please help me?