In response to an opening bid of 1 spade by my partner, why do I have to have at least 5 pieces in hearts (+ the usual points) in order to respond 2H?
In addition to the other two great answers here, I'll expand a bit more on the &quot;why&quot;.
Basically, your goal is to describe your hand as low as possible and as accurately as possible, right?  So you want to show partner whether you have 4 hearts or 5 hearts at the earliest moment possible, while showing other details about your hand too.
If you play 2H shows 5, then you have these choices for most hands (in base 2/1) that are game-interested or game-forcing:
1NT: 6-11, nothing else exciting to say
2C:  12+, Hearts 4 or less, Diamonds 4 or less, Spades 3 or less
2D: 12+, 4 or more diamonds, Hearts 4 or less, Spades 3 or less
2H: 12+, Hearts 5 or more

Plus the various &quot;spade support&quot; options, of course.
Now you've told your partner (who's mostly in the drivers seat) what you have without taking up almost any space.  Your partner can then tell you their next most interesting thing: whether that is a 4 card heart suit (by bidding 2H), can explore NT, can explore all sorts of things, especially over 1NT or 2C, which are the &quot;least informative&quot; bids.  This is a generally good idea in bidding - the higher the bid, the more space is gone, so the more information you need to convey!
2H steals all that space; after that, what can P bid?  2S to show longer spades or maybe if they're stuck, 2NT to decline but not leaving a lot of room to explore NT; or 3 level bids.  Ick.  If you're going to have to be in a long conversation that might end up with having to really think hard about how safe 3NT is over a 5-2 4S, you want to have that conversation at the 2 level, not the 3 level, as you have only so many chances to show stoppers or shape.