A power amplifier is an electronic amplifier designed to increase the magnitude of power of a given input signal. The power of the input signal is increased to a level high enough to drive loads of output devices like speakers, headphones, RF transmitters etc. Unlike voltage/current amplifiers, a power amplifier is designed to drive loads directly and is used as a final block in an amplifier chain.
The input signal to a power amplifier needs to be above a certain threshold. So, instead of directly passing the raw audio/RF signal to the power amplifier, it is first pre-amplified using current/voltage amplifiers and is sent as input to the power amp after making necessary modifications. You can observe the block diagram of an audio amplifier and the usage of power amplifier below.
A stereo power amplifier or an audio power amplifier is a device that boosts an audio signal so that it can be heard through the speakers. Thanks to it, one can clearly hear the music or the audio - without it, the sound wouldn’t be much louder than a whisper, and many people wouldn’t be able to hear anything at all.