It could mean many different things. It could mean that the headspace is minimum. It could also mean the neck diameter is smaller than usual in the chamber. It may also mean the throat diameter is smaller or the base diameter of the chamber is smaller. Tight is not always the best though.
More generally referred to as a tight neck chamber.
A tight neck refers to the fact the there is very little difference between the outside diameter of the neck of a loaded round and diameter of the neck chamber. Using easy numbers as an example, the bullet diameter is .243 and the neck wall thickness of the brass is .013. So the outside diameter of the loaded round is .243+ .013+ .013 = .269. A .271 neck chamber would be a tight neck as there is only .002 difference or .001 from the outside of the loaded round to the neck chamber wall.
Most bench rest shooters use tight neck chambers to help improve accuracy bc everything lines up more precisely. But they also neck turn their brass to control and maintain the tight tolerance. Gunsmiths will generally stamp the neck diameter on the outside of the barrel to identify it as a tight neck.
SAAMI specs call for a greater difference, say .007, to take into account differences in brass thickness and dirt that may get into the chamber that could cause over pressure problems.