The idea is to put yourself in advantageous situations more often than putting yourself in disadvantageous situations. Think of it like a football game. Is starting at your own 10 yard line a good situation? No, not really, but a team can still score if they are good enough. They probably won't do it on a regular basis though. However, that same team starting near midfield will have a much better chance of scoring on a regular basis.
That's the difference between playing premium hands and putting yourself in an advantageous situation, and playing garbage and starting off in a bad spot. Oh, I'd like to point out that playing "premium" hands is largely dependent on each person's style. Some may consider a hand worth raising with, while others will routinely fold the same hand in the same position.
As dj said, there is more to it than just what we mentioned, but it all boils down to setting yourself up for success, as opposed to throwing a hail mary hoping to get lucky.