All sounds like a foreign language doesn't it?!? I'm a noob as well and have done a stack of reading to make sense of all this. This YouTube give a quick am dirty guide to what it means:
https://m.youtube.com/watch?v=35E9Lqjypz8
Basically, an equatorial mount (EQ Mount) is best for tracking stars for happy snapping. The EQ3/4/5/6 etc refers to a model and when boiled down to it, is all about how much payload they can carry. Not surprisingly, 3 can carry less than 5, which is less than six. Again not surprisingly, the 5 is a good mid way so if you decide to start hanging scopes and whatnot off the mount, you have some future proofing to do so.
Being new, go for something like an NEQ5 Pro, which means there's a fair bit you can get the mount to do for you without having to do a masters in Astronomy first (they are called goto mounts because you tell the mount to slew (go to) a star, nebulae, cluster etc of choice.
Second hand is somewhere between about $500-$1500, NEQ5 Pro second hand should be looking around the $500-$600 or so mark.
Hope that helps