Can anybody please explain what is mW rating in audio video transmitter and receiver ?
I saw an A/V transmitter and receiver of 5.8Ghz 300mW.
What is that 300mw ?

Welcome to the forum Arjun. The mW rating refers to the power of the transmitter radio output in milliwatts. Greater power generally leads to greater range though there is a legal limit to the maximum power of radio transmission.

In addition range is not only dependent on the power of the TX but also the kind of antennas used on Tx/Rx.

For ex people are getting a range of 300-500meters with stock antenna(im being generous) on a boscam 200mW 5.8ghz Tx/Rx, while some hobbyists with DIY antennas have extended the range to more than 5kms. One gentleman got 24km range out of 200mW,5.8ghz tx/rx but that would need lot of effort + research + mixing and matching stuff.

http://www.rcgroups.com/forums/showthread.php?t=1441664

As they said the range is proportional to the power rating, and on the other side it also shows the current consumption of the system, the greater the power rating the more current it takes to keep it working!! for you to choose a system keep both range and current consumption in mind!! for normal FPV go with a 200mW and cloverleaf antenna but for some long range go for 600mW and the same antenna!!

regards,
JK

Take it this way. 5.8 ghz, 200mw should give u 1 Km while ul need 800mw to get 2km with same setup. U can also get 20kms with 200mw if ure doing things right.

over heating is the number 1 issue with high power tx(or they need good Cooling system)

Feeding too much for a newbie is like scaring him away.

To put things simple, it is the horsepower of radio transmitter/receiver/transceiver. It is mostly measured in [Watts, in SI units], mW / milliWatts 1/1000th of a Watt, is the measure of power of a radio system.

And welcome to the forum, Arjun18..

It's obovius watts/mw is a measure of power output of anything not only radio. rf equipments also use the same units.

Range is not proportional to the mw rating  of a video tx