An overcall is a bid made after the opposition have already opened the bidding. Just because the opposition have opened the bidding doesn't mean you can't bid. In fact, with the right hand, it's good to enter the auction. You might get to play the hand, you can take up some of the opposition's bidding space and you can suggest a lead to partner.
Good 5 card suit, 10 - 15 points
Unlike an opening bid, points don't matter so much for a simple overcall. A good 5-card suit is the key and good rule of thumb is two of the top four honours in the suit.
This next hand is not strong but spades would make a good trump suit, you take up opponents' bidding space and, if you end up defending, partner will know to lead a spade.
A good 5 card suit is all you need for an overcall
After an opening bid you will normally respond with 6+ points but you're not obliged to respond to an overcall in the same way. Sure, sometimes you'll want to explore for game and occasionally even slam possibilities but normally you'll simply be looking to compete for the partscore.
An opening bid of 1♣, 1♦, 1♥ or 1♠ doesn't promise a good suit. If you're playing 5 card majors, an opening bid of 1♣ or 1♦ might even be a three card suit. You'll be responding to an opening bid with 6+ points or even less in some cases in order to search for the best contract.
Partner's diamonds might not be very good and you might have a game on. Bid 1♥.
Unlike an opening bid, partner's overcall does show a good suit. There's often no need to search for a better contract because you're likely to already be in the best spot!
Partner has 5 good diamonds. Pass!
Remember that an overcall won't be a strong hand so you're very unlikely to miss game by passing.
If you do have a fit, it's good to support partner. 3-card support will be enough, because partner is showing a 5-card suit.
Points don't matter - show your trump fit!
Good 6 card suit, 6 - 10 points
A weak jump overcall is similar to a weak 2 opening bid. You can quickly describe your hand to your partner and at the same time make it hard for the opposition to enter the auction
Good 7 card suit, 6 - 10 points
A preemptive overcall is similar to a preemptive opening bid.
An overcall of 1NT shows 16-18 points, balanced.