"You can't beat ___" means "___ is the best!" For example:
When it's hot outside and you want to relax, you can't beat a nice, cold beer.
The word "beat" here is used in the sense of beating someone in a competition. If something is clearly the best, that means that you "can't beat" it.
You might also see this expression used in the form "___ can't be beat". For example, in a television commercial:
Our low prices can't be beat!