Dexcom is a wearable glucose monitor, @Deathraystare - it's a little module that goes on your arm or abdomen, with a tiny needle that goes into the skin and allows it to measure your blood glucose (from the fluid in the cells, not directly from the bloodstream). It gives you a regular reading, and it will sound an alarm if your blood glucose goes too high or too low.
I use the Dexcom 1, but there are other Dexcom models, and FreeStyle Libre also do a similar wearable monitor.
If you go to the Dexcom and Freestyle Libre sites, you can get a single monitor to try out, for free, just by entering your email address. The Dexcom 1 works for 10 days before needing replacement, and the Freestyle lasts a week, I think.
If you are type 1 diabetic, the NHS may provide a monitor, but as a type 2, I have to buy mine - they aren't cheap, but for me, it is useful and reassuring to know that the medication and diet changes are improving my blood glucose levels.