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 3bet and the button calls 3bet, the small blind will have the initiative. In principle, 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 received lessons to improve your poker skills, you've been told about the importance of playing in position. This is obviously important because you will make more money playing in position than playing out of position. But another key to winning poker is to have initiative. When you have lead or initiative, you can represent 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 as 55 and your opponent calls with 77. So you have lead. The flop comes A-K-Q. Your opponent check, you bet and he lays down the best hand, simply because you have the lead and the flop might very well have hit your range.