In 1967, Bobbie Gentry was on top of the hit parade with her "Ode to Billy Joe," where she lamented the sad, sad story of one of life's losers.
Today, we're going to find a way to NOT be among the Billy Joes by using the Law of Total Tricks. There are huge books written on The Law, but let's summarize. This isn't the whole story by any means, but mastering this much will put you ahead of the pack.
Basically, The Law says that in a competitive auction, when you have a weak hand, raising your partner's suit shows a WEAK hand, and the level you raise to shows the number of trumps in your combined hands.
In other words, with a weak hand, you're bidding the number of TRUMPS rather than the number of POINTS.
(And what do we do if we have a fit with Pard and a NICE hand? Story for another day.)
Huh? Example: Your Pard overcalls 1♥. You know Pard has at least five Hearts. You've only got 5 HCP, but you have four Hearts. You've got nine combined trumps, so you JUMP to 3♥. This shows weakness and four Hearts. You bid one trick for each trump. Let's practice and it will become clearer.