They have a small filament that sits in your arm and has a round disc on the outside that sends the readings to your phone.
You don't feel anything but you will need to hold your phone over the sensor at least every 8 hours to down load the data.
The Libre, which is well known one, costs £50 each 2 weeks as the sensors last 14 days. You don't have to use them continually. I just use one on holidays and whenever else I feel I need to keep a better check on myself.
Once you have learned what foods suit your body and which don't, you won't need to test very often, if at all.
The alternative is to use a meter that tests a drop of blood. It takes seconds to do and can be discreet. Much cheaper to buy, the meters are often free as you just pay for the testing strips