I was looking at propellers for my quadcopter, and I saw that some props like the Gemfan brand on eBay all had a 5030 number or 5040 number in the item specifications. Do those numbers mean like the stiffness of the props?
They indicate the size and pitch of the props, 5040 means 5" prop 4" pitch. 5045 means 5" prop 4.5" pitch.
The pitch is the theoretical distance a prop could travel in one revolution.
For a beginner I would suggest you try the new "unbreakable" props from surveilzone. http://www.surveilzone.com/high-quality-customized-5040-multirotor-propellers-two-pair-cw-ccw-g-1352
They are not as efficient as some of the others but they are apparently extremely robust.