For a lot of players there's a weird grey area in the basic fundamentals of fighting games. When we start out we tend to do things like jump and dash at the opponent a lot, but we quickly learn that human opponents can easily counter these things with consistency if their opponent does them blindly.
Some funny things happen after this, depending on the player(s). Some players get it into their head that these actions are invalid and stop doing them altogether. Some players are the opposite and keep doing them mindlessly. Some players start getting hit by jump-ins and get rushed down by midscreen dashes then wonder why they can't stop it, but the opponent can. And then some players realize the truth behind it, which is baiting.
These concepts (like anti-airs > jumping) are not set rules at all. A jump does not always necessarily mean that the person jumping should get hit. The reason is because players can bend and break the rules using other tools.
Leading is a word that I just randomly decided to use as a label for this particular subtype of baiting. It's generally just called baiting or footsies. But, I feel it's significant enough to pay it special attention, since it's very fundamental to almost every fighter.
I'll use SF3:3S for the sake of examples though, for whatever reason. In this case, Dudley and Ibuki. You don't really need to know much about these characters other than that Dudley has a very low and fast jump with good jumping attacks, but Ibuki has extremely good anti-airs and very low and long ground pokes. Thus, if Dudley jumps or dashes blindly then he will be hit on reaction very easily.
Therefor, leading is what Dudley must use to get in on an Ibuki player that is stationary and waiting to counter him. To do this, he has to walk within poke range of Ibuki and try to fish out or provoke a reaction out of the player. That means stepping into a suboptimal range and basically risking being hit at Ibuki's max range.
This is intentional though. By simply walking forward, the Dudley player has applied pressure to the Ibuki player, forcing their hand. Eventually Dudley can keep inching forward and Ibuki will have to take some sort of action to get him the hell away from her. Once that c.MK gets fired off, the rules change!
Ibuki's c.MK animates for 22 frames total (on whiff), and it's also one of the hardest things for Dudley to deal with on the ground against Ibuki, but it can also lead to Ibuki's undoing if properly fished for. Once it's out, Dudley is now "allowed" to jump. In fact, he should jump. Even if Dudley only noticed the c.MK and reacted physically half-way through the animation, Ibuki still won't be able to do anything until the c.MK is done animating, which won't be for another 11 frames (half of the full attack).
Now, 11 frames doesn't sound like a lot, but it most definitely is. Because Ibuki is locked in place there is no hope of walking backwards outside of Dudley's jump range, or walking forwards under him either. Ibuki also can't use her most reliable anti-air either, which is c.HP, because the startup time and positioning required for that is not possible at that point. This also limits exactly when Ibuki can even perform an anti-air, giving Dudley a much better chance of using an air-parry/attack OS. And that is how you change the rules.
This is the same with dashing, too. For the most part, most characters should never ever even dream of dashing towards Chun-Li when she has a full bar stocked and is just sitting there. But once you lead her into whiffing a laggy attack then the chance for her to connect a c.MK into super goes away.
This is why you see a lot of "wiggling" and "dancing" in games like SF3:3S. A lot of novice players just mimic it without understanding why the good players do it, but the above explanation is exactly why they do it. They want to pressure the opponent into slipping up and giving them an opening.