From the website:
4 Pack. This will flash a red light when an air pressure loss occurs. When screwed onto the tire air valve of any properly inflated tire, it will record this pressure permanently. When pressure loss exceeds 8 psi, the LED light flashes. If flashing, add air to the tire, reinstall, and the flashing will stop. Battery life is 2 yrs. 3 weeks with the light on. Pressure range of over 50 PSI. $31.75
Cheap insurance on your horse trailer. Especially good for those trailers that are not used regularly.