What's mcd mean?

mcd stands for millicandela -- it refers to the overall brightness
(technically referred to as the "luminous intensity") of an LED (or
other light source). It's based on the standard for measing light
intensity, the candela. A 1 candela light source produces 1 lumen per
square meter at a 1 meter distance from the source. A lumen is a a little
more complicated to define.. Any good Physics textbook or encyclopedia
should explain the whole thing with visual aids if you're still
wondering.

LED Tire Pressure Alert

Under-inflated tires can cost you gas mileage, tire life, and even your life ... but how many of us remember to check them regularly? No need for reminders: you won't be able to miss the bright red flashing LEDs of these pressure monitor valve caps. The patent-pending system automatically calibrates itself to your tire's pressure (10-50 psi) and starts flashing if pressure drops by more than 4 psi. The completely self-contained units include a 2-year battery.

Features
Flashing LED alerts you to dangerous tire pressure loss.
Under-inflated tires can cost you gas mileage, tire life, and even your life.
The patent-pending system automatically calibrates itself to your tire's pressure (10-50 psi).
Starts flashing if pressure drops by more than 4 psi.
The completely self-contained units include a 2-year battery. Set of 4.