If an opponent opened 1♦, (for example), and you Overcalled 2♠, you were showing a stronger hand than a simple 1♠ Overcall would have shown.
The problem with that convention is that it doesn't arise very often. Modern players have found a better use for the jump overcall, called the Weak Jump Overcall.
This is one of the easiest Conventions to describe:
A Weak Jump Overcall is exactly like a Weak 2-Bid.
- 6-card suit with some honors
- 6-11 points
No lesson per se, but look at the Weak 2-Bid lesson.