The significance of something being in spades is that in contract bridge the suits are ranked in this order:
The top two are known as the major suits, and a trick taken in a major suit is worth 30 points whereas a trick in a minor suit worth only 20; so if you win anything in spades you are getting a 50% bonus, hence the phrase. (It’s true that winning in hearts brings the same benefit, but as spades is the top suit of all (as it is in some other games), it just seems more natural to say ‘in spades’.
There’s also an amplified version of this phrase: doubled and in spades. In bridge, if your opponent makes a contract bid that you do not believe they can succeed in winning, you can call ‘double’, and if that contract is played your opponents will score double points if they succeed, but if you defeat them you will score double. So winning or losing a contract that’s doubled and in spades is a big thing.