Definition of lead
To have the lead is to have the initiative in French. You have the initiative when you have made the most aggressive play on the previous street. If the button opens, the small blind 3-bets and the button calls the 3-bet, the small blind has the initiative. Normally, the player who has the initiative will bet at another frequency on the flop (this is called his cbet). The player who has the initiative theoretically has the strongest hand preflop (otherwise he would have been raised by his opponent).
Having the initiative, a key to winning poker
If you've ever been taught how to improve your poker, you've heard about the importance of playing in position. This is obviously important since you will make more money playing in position than out of position. But another key to winning poker is to have the initiative. When you have the lead or the initiative, you can make hands and you will be given a lot of credit. Coupled with position, initiative is an essential part of winning poker. For example, you open a hand like 55 and your opponent calls with 77. You have the lead. The flop comes A-K-Q. Your opponent checks, you bet and he folds the best hand, simply because you have the lead and the flop could very well have hit your range.