This is an example only. There are many varieties, but this should give you the basic idea.
Lets say you want to know how much acid is in a sample. One way to do this is to directly titrate the acid. Sometimes, there are interferences that stop that.
With a back titration, you add the sample to a known amount of base. The acid in the sample reacts with the base. You then titrate the left over base in the sample with an acid (this is where the "back" titration comes from). From this information, you can calculate the amount of acid in the initial sample.