An amplifier is an electronic device that increases the level of a signal. In Television, an amplifier is a broadband device that increases the amplitude or strength of a TV signal for distributing the signal to multiple TV's.
An amplifier is rated in 3 ways:
Bandwidth: What frequency range the amplifier will amplify
Gain: How much the amplifier adds to the input signal level, expressed in dB
Output Level: How strong a signal the amplifier can output while maintaining performance