This device is sometimes called the "Power Injector Module"

Its sole purpose is to apply DC voltage (from the supplied AC/DC adapter) to the antenna unit.

Note: This is NOT an antenna amplifier. The amplifier is already built into the antenna unit.

See picture:
It is essential that for proper operation of your antenna system, this Power Inserter module (b) is installed at the coax cable from the antenna unit (a). It must be connected between the antenna unit (a) and any devices (c) such as splitters, matching transformers, networks and receiving devices (d) like TV, Sat Box, DTV converter, etc.