Not enough information. If you mean an inline gear indicator, it's a worthless gadget that splices into your rear derailleur cable housing that has a clear window through which you can see the cable. The cable is marked and the clear window has markings so you can see roughly what cog you're on.
Thank you! Sorry for the vague question. I heard that it is a cheap alternative to getting the Shimano Flight Deck that has a selling point of having a gear indicator on it. Glad Scooper and Ruppster interpreted my question correctly!
I think I'm going to get one. I have a triple chainring (not ashamed to admit it!) and sometimes I absentmindedly stretch the chain across the rings.