This is getting to be such a common question that I'm tempted to learn the answer myself. Here's a pretty decent video I just watched about the early moves of the Ruy Lopez:
Chess openings - Ruy Lopez
https://www.youtube.com/watch?v=_Bs7UtQR58Y
It might be easier if you wrote specific moves you wonder about, how far into the game you mean, and in which variation. Personally, I'd be interested in a nice overview of the main variations of the Ruy Lopez, each of which probably has its own goal.
Every chess opening has a mian objective. Some positional advantage or attack that you're trying to achieve. It is the opponent's dismantle your attempt.
So what about the Ruy Lopez. This is my favorite opening to play as white but to be honest I sometimes find myself at a loss for an overall objective. I get caught up in little tactical battles with no real vision for what I am trying to achieve in the long run.
What are white's main ideas in the Ruy Lopez?
What about black?