What's an Overcall? It's a bid by your side when the other side has already started. You are calling over an opener's bid. It means the bidding has become "competitive" with both sides vying for the contract. To overcall, you need a good suit with five+ cards containing at least two of the top three (AKQ), or three of the top five honours (AKQJ10).
Making an overcall at the one-level is a very good idea. You don't need an opening hand; in fact approx 9 points is fine, provided you have a good suit, and most of the points are in that. It may be strong too, up to 17 points. Even though you might have opening values, you're known as the "overcaller" because someone has already opened (and there's only on "opener"). Partner doesn't have to reason to an overcall; it's not a forcing bid.
There are three reasons to make an overcall. (1) It might help partner find the best lead if your side ends up defending. This is possible as the points will be evenly divided between the two sides. (2) You might upset the opponents‘ bidding (3) You might make your own contract.
The hand shown above fits the bill for overcalling 1♠ , and even with one fewer club honour, would have been strong enough to overcall at the one-level. Partner raises your suit, as if you’d opened.
There's a difference between overcalling at the one level and the two level. You need an opening hand to bid a minor suit after the opponents have started with a major. You don’t yet know how strong opener’s partner is. It could be dangerous without a good hand. Here, the South hand would have opened the bidding, but because East has already started with 1♥, you need a good club suit, as well as opening values to bid at the two-level.
East’s opens 1♦, and South overcalls 1♠, with a good suit, and 11 high card points. This shouldn’t stop West from showing a diamond fit and 6 – 9 points. North raises partner’s overcall to 2♠. This is where the bidding finishes. Sometimes, making a good partscore is the best you can do.