Eventually one of them drops out of warp near you. Even in an MMO, it's a very odd feeling having another player ship drop out of cruise, and slowly pull up to your windshield.
They say hi, did I find the right place, you need fuel right? They transfer a bunch of fuel, give you pointers to the nearest systems with fuel services, and make sure you're in good shape to get yourself to safety.
The thing I find remarkable about this, is that the Fuel Rats aren't a game feature. There are no rewards for being a fuel rat (in fact, it's policy to refuse payment if the recuee offers), except the intrinsic reward of helping people have a nice time.
Despite this, the fuel rats were one of the first "guilds" formed, if you want to call them that, and have been going strong for years. According to their stats, they have rescued 157 thousand stranded players, with a 96% success rate.