it measures something or other in your urine and predicts when your fertile days will be. works best after your first month tbh. it gives you a clear indication of when would be the best time to ttc. Works a bit like a a basal temp. but with one of those you have to wait till your temp peaks to know if you are at your fertile time, with a clearblue it can predict your fertile time so that you can have the best chance of falling pg by had a sex over the total period of your high fertile stage
It does the same thing as the Persona monitor, but reverses the alerts, IFYSWIM.
It tests your urine on certain days of the month, and detects the amount of lutenising hormone in your sample. This hormone surges just before ovulation. It then lights a symbol to tell you you are highly fertile.
The Persona monitor is intended for contraceptive purposes - green days you can freely have sex, red days are highly 'dangerous'. So, to conceive using it (as I did accidently with DD1, and deliberately with DD2), you have sex on the red days.