GH (general hardness) is the amount of dissolved minerals — mainly calcium and magnesium — in your water, and it matters more than pH for matching a fish to your tap water.
Some fish want soft water (low GH: many tetras, rams), others want hard water (high GH: livebearers like guppies and mollies, most snails and shrimp for their shells). Working with your tap water's natural GH is far easier than fighting it.
GH is a big reason a fish "does fine" for one keeper and struggles for another on different tap water — check yours before buying.