the first number or range of numbers is the power.
for example
2-7x40mm. that means you can turn a ring on the scope and adjust it from 2x magnification up to 7x magnification.
6.5-20x40mm is 6.5-20x magnification with a 40mm objective.
each power will generally have a field of view (F.O.V.) listed alongside it that will let you know how wide an area youll be able to see at each magnification.
the second is the objective size. the 30mm range is compact, 40mm is pretty much standard these days, and 50mm-56mm is a bigass scope. the larger the objective generally the more light the scope will let in and the brighter it will be in less ideal light conditions. generally 40mm is the highest people go for hunting use unless they intend to be making many fading light shots, since the 50mms are quite bulky, generally heavier, and are harder to mount low over the bore.
if you want something adaptable for general hunting use make sure it has a
low power setting in its power range, something like 2.5x, 3x or 4x maximum. ie: dont get a scope with a rediculous range like 10-40x and expect to use it for hunting anything but varmints
thats it in a nutshell. i highly recommend you go and read the optics forum for a day or two and then go out and actually handle the scopes you want to buy (avoid salesmen in dept. store style outdoor shops since they generally dont know wtf theyre talking about) before you drop $1000 on a scope you know very little about. there are other variables in a scope and scope mounting such as tube size, AO, types of turrets and reticles, etc that you should know at least a little bit about and are covered here in the forums.
here is a very basic rundown of the different terminology youll find in scope specs:
http://www.nightvisionplanet.com/Articles.asp?ID=124
i havent read the entire thing so if something in there is wrong dont blame me
good luck

youll love the Sendero, theyre a great rifle