A relay buoy dropped by Enterprise NX-01 along its course to boost subspace signals, allowing the first deep space explorers to keep in real-time contact with Earth.
Subspace amplifiers are relay buoys deployed by Earth's first warp five ship to extend the reach of its communications. They are a forerunner of the subspace relay networks of later centuries.
In the Enterprise episode Silent Enemy, the ship launches two of them, named Echo One and Echo Two. Archer uses the improved link to speak to the families of his officers, and Hoshi Sato draws on it in her attempt to discover Malcolm Reed's favourite food for his birthday.
The same episode sees an unidentified alien ship attack Enterprise repeatedly, and one of the amplifiers is destroyed. The link to Earth also lets Archer consider returning home to have the ship's phase cannons installed, before Reed and Tucker fit them on their own.