There are other threads you can search that have a more thorough discussion on this topic, but let me take my best shot at helping you out with your question. The combine score compares the players results in the different combine events with their position, height, weight and speed to yield a cumulative score. It is also important to know how the different test correlate to actual ratings.
40 time = speed
Bench and Broad Jump = Strength
Vertical = Jumping
3 cone and 20 yd shuttle = agility and acceleration
This link gives you a breakdown of the 40 and bench equivalents in terms of ratings
http://forum.ea.com/eaforum/posts/list/10354056.page
However, this cumulative score can be misleading, because you might not care about certain stats in comparison to others for a given position. For example, if I am scouting a DT and he runs a good 40, the best in the class, but does awful on the bench, I want no part of him. And conversely maybe he has a great bench but a dreadful 40, that is just fine with me.
So to answer your question more directly, after the combine scores are released, I like to take a look and note the extremes from each position group, the fastest qb, rb, wr, etc. Just like in real life, these numbers are not the be all end all of what makes players good, but as we know it is much harder to upgrade physical attributes in comparison with skills so if two players are close, it can be a valuable tie breaker.
The last point I would like to make was discussed in my post on the biggest freaks/studs/gems. Look for unique traits that might make a player translate well to another position. For example, if you are looking for a speed rb, you aren't going to find one in the draft, the fastest you will ever get is maybe a 92 speed. The best way to find one is find a very fast WR (4.3 forty or less) with excellent 3 cone and 20 yard shuttle and convert them (which you can do right after you draft BUT BEFORE YOU ADVANCE TO THE NEXT SCREEN. Other great position switches are OLBs to DEs or OLBs to SSs and CBs to FS. Because the game naturally creates very slow safeties.
I hope this helps!