tolerance stacking

I'll give you two answers: the first to what you asked; and the second to what you may mean.

Simple tolerance stacking is straightforward. Imagine you have four 6mm holes in a row. You have a dimension of 20 (+/- 0.1) mm between each hole. By the time you get to the last hole, it could be 60.3 mm from the first (20.1mm + 20.1mm + 20.1mm). That's simple stacking.

Tolerance stack-up analysis is a much more complex operation wherein you consider all tolerances, location, concentricity, parallelism, MMC, etc) to arrive at whether things actually fit together. It generally requires some sort of software (there are specialized stack-up analysis packages and you can do it, sort of, with other packages). It's absolutely maddening and doesn't necessarily tell you anything useful. It's great for manufacturing auto transmissions but way too complicated for a trailer hitch.

Here's a link to a brochure for a course offered by the University of Wisconsin at Milwaukee. It gives a better overview than my brief answer.

http://www4.uwm.edu/sce/resources/eng/uwm_tolerance_dimension.pdf