Yeah, kinda like when someone says a knife feels "alive" in your hand. Balance, weight, ergonomics, etc. all come together like the planets and stars aligning and you have such a feeling of control of said knife. Feels like it's a natural extension of you.
Or, they could mean deployment speed, etc. as others have said. But I think if you hear the term "quick in the hand" it's usually talking about that "alive" thing I was talking about. At least that's how I see it. Hope this helps.
Gibby