Weak means strong, strong means weak. Age old poker tells that were recited by Mike McD in the movie Rounders while the table of New York City's card sharks decided to carve up the tourist at the Taj in Atlantic City.

While playing lower stakes and in most poker games, a person's bet means exactly what they're holding. See that pretty Ace of spades on the flop, and the once sleepy seven taking a time out from stirring his coffee and rifling chips while placing a pot sized bet? He's probably got at least a matching ace in his hand.

Two hearts on the board and someone calls your pot sized bet? Put him on a flush draw. But, many players go beyond these simple tells and bets while confusing their opponents to their real holdings. A smallish half pot sized bet on a ragged board? Many aggressive players would raise you with any two cards. Unfortunately for them, your flopped straight might prove to be a little stronger then the shark's ace high.

To drive home the point of the "weak lead" bet, here's Lee Watkinson from Full Tilt Poker to talk about taking advantage of that "pro" player's aggressive tendencies while extracting value from your monster hands while betting less:

Have you ever seen those nature shows where scientists drag a fake seal behind a boat so they can provoke sharks to attack? The form of the helpless seal is irresistible to sharks, and they attack nearly every time. In a way, some professional poker players are like those ravenous sharks: they're looking for easy prey. They're searching for signs of weakness that will allow them to pick up pots with little resistance.
These pros can make life very difficult for an amateur. But there are ways that an amateur can take advantage of a pro's aggressive instincts. One useful play is called the weak lead.
Using a weak lead, a player bets out a relatively small percentage of the pot. For example, if there's 800 in the pot, the amateur would put out a bet of less than half - say 300. Looking at this bet, an aggressive pro may assume that his opponent doesn't have much of a hand. He may guess that the bet indicates some sort of modest connection with the flop or even a poor bluff. The pro may look at this bet and come over the top with very little, or maybe nothing at all. Knowing that a player is on the lookout for these sorts of signals means you can offer up a weak bet when your hand is actually very strong.
Say you're in a tournament and you raise with pocket Jacks. The pro calls behind you. The flop then comes 2-6-J rainbow, giving you a very strong hand. This is a great time to put out a weak lead. A small bet may entice the pro to believe you have something like A-K and missed entirely. He may raise right there, at which point you can decide to either call or pop it again. Or he may call your initial bet with the hopes of taking you off the hand on the turn. You can then check to the pro on the turn, and be all but certain that he's going to bet, at which point you can put in a raise.