The inactive ingredients in meds are mainly used for the composition of the actual tablets. For individuals with diabetes, it's important that they watch for sucrose or glucose. For people with celiac disease, they have to make sure nothing has gluten, wheat, or starch.
The inactive ingredients are basically called "fillers". They are what help to keep the active ingredients (the actual medicine) mixed well within the tablet/caplet.
For most people, they don't have any effect. Some people are allergic to the dyes that color the tablets, while others are allergic to some of the other fillers.
Hope this helped