To have a merged or polarized range -- this is generally something that is said regarding a betting/raising range, particularly a 3-betting range preflop.
When 3-betting, the first question you must ask/answer is: Why am I 3-betting?
1. One reason to 3-bet is that some people fold too often to 3-bets. So the reason we are 3-betting is to make money off their folds. Against such a type, we generally want a polarized range. So, depending on how much they fold, we could construct a polarized range that looks something like this:
Value hands: AA, KK
Bluffs: A2s-A7s, A6o-A9o, K7s
PLEASE: Do not take this range seriously, I didn't spend any time working on it, I just made it off the top of my head.
Anyway, so this range is pretty ridiculous, as we have 12 value combos of the nuts (KK+), and like 76 or something combos of bluffs (this ratio probably isn't going to work out well in practice against anyone). But this is mega-polarized, and is ridiculously weighted toward bluffs. (Against someone who is folding way too often vs. 3-bets, we might want to start flatting AA/KK once in a while so as not to lose value with those hands. But then our range is so ridiculous, someone -- either villain or someone at the table -- is going to notice that our 3-betting range is ridiculous in this spot, and they can exploit us)
(If you want to know why we chose these hands to bluff with, it's sort of another topic, but the quick and dirty version is this: Because this guy is folding so often we want blockers to his continuing range, which is always going to include AA, KK, AK. Since we print money vs. this guy because he folds too often, we don't care what our flopability is. Vs. someone with a 20% opening range, A6o has the same hot-cold equity as QTs. But A6o is a better hand to bluff with because A. It has blockers to his continuing range and B. We can profitably flat call with QTs whereas A6o is one of the better hands we would fold.)
2. Against someone who opens wide and never folds to 3-bets, we would 3-bet him with a different range. We want good hands that play well vs. his range, hands that flop well and make good top pairs. So we use a merged 3-bet range vs. this type of villain, and it might look something like this (again, don't take this specific range too seriously):
Here, we are not bluffing, but we are reraising a super wide value range.