Sorry, I didn't catch the title when I originally read this somehow. A belt transect is basically an extended version of a quadrant, which used to measure population density and distribution. Usually a rope is placed from point-A to point-B and a specific distance is set on each side (or some use two ropes) to account for the area. The organism(s) of interest can then be counted if they fall within the parameters and density can be calculated. Sometimes they also measure the distance of each organism from a point of interest as well. In this way they can analyze their distribution. Usually this technique is used to count larger organisms, a lot of the time for trees and vegetation. I'm sure it can be used in both terrestrial and aquatic applications though.
If the question is asking changes in species distribution.. I would also assume that they are either talking about the distribution from a particular point of interest, or change over time. I'm not quite sure. I don't think they would mark you off if you addressed both.