There is no cut and dry definition for tight controls. So while sandalfoot's answer is not wrong, I would give it a different answer.
Tight controls mean less momentum affecting change. If a character is moving in one direction, turning them in another direction very rapidly would be tight controls. If you need more time to get them moving in the desired location, the controls would be looser.
Let's take FIFA for example. If you change directions 180 degrees, it takes time to stop and gain momentum in the other direction. If it was instantaneous, the controls would be too tight. If it took too long to turn around, the controls would be too loose. You can also turn in a semi circle to maintain your momentum, while taking longer to turn around. The momentum lost while turning this semi circle can be adjusted to change the tightness/looseness of the controls. Or for other games, such as Mario, how drastically you can change your direction while mid air reflects the tightness/looseness of the controls.
You do not want tight or loose controls - you want balanced controls. If they are too loose, and it takes too long to change direction, the game can feel unwieldy. If they are too tight, and you can immediately change directions, they can see unrealistic. You want to hit a middle ground.