If a visitor goes to your site and generates several conversions of a given type (for instance conversion 1) we have two options : we can count as many conversions as were generated (so if the visitor goes to your purchase confirmation page 4 times in one session we count 4 conversions) or we can count the fact that the visitor converted during this session as a single event, in which case we would record one conversion only, no matter how many times the visitor hit your purchase confirmation page during the session.
Use this setting to tell us what your preference is: the 'Multiples' option counts all conversions. The 'Uniques' option aggregates the conversions and counts a maximum of one conversion of a given type during a visit
Note that if you use the 'Uniques' setting and the visitor generates conversions of different types, we will still count one conversion per type. So for instance, if the visitor generates 3 conversion 1 and 2 conversion 2, we will count conv1 = 1 and conv2 = 1 for this visit.