Throat hit refers to an almost "scratchy" type feeling in the back of your throat when inhaling vapor. It's the feeling you sometimes get right before you start to cough.
Some vapers prefer a more intense throat hit, as it has a kind of satisfying feeling, and is reminiscent of the feeling you get in your throat when smoking a conventional cigarette.
Some vapers don't want any throat hit, because it makes them cough, which can be pretty annoying.
The amount of throat hit you feel is determined by 4 things:
1) Your PG level. Higher PG = Stronger throat hit
2) Your nicotine strength. More nic = more throat hit
3) Your device settings & capabilities. High wattage? Low resistance? High temp? More power? All of these things will contribute to a stronger throat hit.
4) How you breathe! The longer you inhale, the more vape you'll feel at the back of your throat. Plus, if you take lung hits (inhaling directly into your lungs), you'll experience a stronger throat hit than if you mouth-to-lung inhale (hold your vape in your mouth before inhaling into your lungs, like with cigarette smoke).