If you make an overcall that uses up some bidding levels i.e. a jump to an unnecessary level e.g. 1♥ (3♠), it shows a weak hand (6 - 9) with a long suit, just like a preemptive opening. You need all the honour cards concentrated in the long suit, and very few points outside that. These are Preemptive Jump Overcalls.
This action often muddies the waters for the opponents, who may have a lot of points and be trying to find a fit. When you overcall preemptively, you’re not expecting to make your contract, but hoping that the penalty for going minus is less than the score the opponents could have made.
Because you might be doubled for penalty, you need to have around six playing tricks to bid like this, e.g.
♠7 ♥KQJ10853 ♦75 ♣732
This hand, with the seven card suit ♥KQJ10853, missing only the ♥A, is worth 6 playing tricks all by itself.
The more cards in your long suit, the higher you should jump. That’s how a preempt works.
Partner doesn’t have to bid (an overcall is not forcing), but they should raise your suit if they hold a few trumps. There’s not much point in bidding their own suit when you’ve told them you have only one place to play the hand.
Hands with big fits and few points are called offensive because they will only do well if their suit is trumps. Defensive hands contain honour cards in other suits, and you expect to take some tricks in defence if you don’t get to declare.