An overcall is a bid you make when the opposition have already opened the bidding.
On the following hand East has opened 1♥ and I'm going to bid 1♠. So my 1♠ is an overcall.
The most important thing to consider when you're overcalling is the quality of your suit. Here I've got a good 5-card suit. And that's the type of hand that you really want to be overcalling on.
Let's have a look at a few more examples.
This time, East has opened 1♥ and I have a 5-card spade suit but I've only got one honour and it's not such a great hand, so I'd rather pass.
East opens 1♥ and this time, I'm going to overcall 2♦. I have to go to the two level but I've got a really nice diamond suit, so that makes it worthwhile. Even if I don't get to play the hand my partner will know to lead a diamond, which I'll be very happy about.
There are a few reasons why you might want to overcall. The most obvious, of course, is that you get to play the hand. An overcall can also help if you end up defending the hand because it'll tell partner what to lead. And an overcall can also be a disruptive bid. Even a simple overcall can take away bidding space from the opposition and make it harder for them to get to their best contract. Finally, of course, it's fun to bid, so get in there and have some fun.