An overcall is a bid made after the opposition have already opened the bidding. Whereas an opening bid can show a 3, 4 or 5 card suit depending on your bidding system, an overcall is always a 5 card suit.
Good 5 card suit, 10 - 15 points
Unlike an opening bid, you don't need to have 12 points 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
Weak jump overcall
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.
A 1NT overcall must also have a stop in the opponents' suit.