A 'Hand Range' is the range of possible hands that your particular opponent could be holding.
Let's consider some useful terminology:
(On any street before the river it is frequent for the bluff part of a polarized range to be consistently stronger than the worst possible hands. For example preflop, when people 3bet a “polarized” range they typically 3bet bluff with hands that are decent but not strong enough to call i.e something like Q9s, but usually never something like 27o. For this reason, to an observer a polarized range may sometimes be perceived as depolarized/merged. Once the river is reached , a truer polarization effect is observed where a good player will bet his absolute worst hands and absolute best hands.)
Occasionally you may also see players (good players even) describe someone's range as being "polarized toward" a specific type of hand. Eg - "His range is polarized toward one pair hands". This demonstrates a misunderstanding of the word "polarization" which implies polar opposites, yet amongst many circles it has become accepted terminology nonetheless, and means the same as “weighted towards”.
Depolarized/Merged – A merged range is a range arranged in a linear manner. If someone 3bets the top 8% of hands they are considered to be 3betting a merged range. It usually describes a value range with no bluffs.
The expression"merging your range" typically applies to river play. It describes when a player bets a slightly weaker hand for value in a situation where they are perceived to be polarized (and therefore not have that weaker hand in their range) in the hopes of getting called by a worse bluff-catcher. Fish unintentionally merge their river range all the time by over-valuing hands that wouldn't typically expect to get 3 streets of value.