The Supply Signal will summon a cargo plane to deliver an airdrop at the signal's location. The plane will drop 1-3 packages in any direction, so be careful when throwing them, especially when close to mountains.
As of yet, it is not possible to research or craft a Supply Signal.
Smoke signals can be found in airdrops. If you're lucky, you can recieve smoke signals from the airdrop and call multiple airdrops in a row for a sizable period of time - this will attract almost the full server to your location.
This item can be both a blessing and a curse. If you drop it and nobody is around, It's a great item to have. But if there's players on, especially on PvP servers who'll actively seek out airdrops for their chances to spawn good weapons and explosives, you could get hunted down and killed. Your smoke signal will have been worthless.
It is NOT advisable to use this item anywhere near your house. Rust has a 'clan' population, meaning that large groups of people may approach your base wearing suits of full Kevlar and may possibly raid it as well as stealing your air-dropped crates. Sometimes this isn't a bad thing, as air dropped crates will contain things of value, but killing a brave player will result in probably more valuable items than the airdrop itself.
Deploying this in the wastelands is a good idea. Very few players live there, and other players will have to do a lot of running to reach the airdrop, leaving you time to disappear.