Wikipedia has a quick overview:
Guanyin is an East Asian deity of mercy, and a bodhisattva associated with compassion as venerated by Mahayana Buddhists. The name Guanyin is short for Guanshiyin, which means "Perceiving the Sounds (or Cries) of the World". She is also sometimes referred to as Guanyin Bodhisattva.
It is generally accepted among East Asian adherents that Guanyin originated as the Sanskrit Avalokiteśvara. Commonly known in English as the Mercy Goddess or Goddess of Mercy. This bodhisattva is variably depicted and described and is portrayed in different cultures as either female or male. In Chinese Buddhism, Avalokiteśvara has become the somewhat different female figure GuanyinI'm not really sure why, but there's a deep resonance here. I'm in a place in life of serving, of providing nurturing and support to others even as I am unable to partake of that myself. It's happening in my yoga practice - even as my skill and popularity as a teacher increases, I find it a struggle to practice except on my own. I teach the practice I want to take but find it so difficult to connect with the practice as led by others. Similarly, I am the cheerful and kind voice of the Christmas season for clients.
According to Mahāyāna doctrine, Avalokiteśvara is the bodhisattva who has made a great vow to assist sentient beings in times of difficulty and to postpone his own buddhahood until he has assisted every sentient being in achieving nirvana.