Born from a tear drop of Chenrezig, and a vow to be a mother to serve all sentient beings until they reach Buddhahood, the female bodhisattva Green Tara is one of the most well known and practiced figures in Tibetan Buddhism.
The twenty-one manifestations of Green Tara each represent a certain aspect of sentient beings' fear and suffering in order that people can pray and honour Tara for protection and liberation from different fears. While Tara's relative blessing is protection from fear and suffering the ultimate blessing is to become Tara. Written by Buddha Vairochana, a very well known and well-practiced Tara praise is the 'Praises and Prostrations to the Twenty-one Taras'.
Contained herein is a detailed commentary on this by Chamgon Kenting Tai Situpa. Rinpoche goes through each of the twenty-one Taras, describing how each appears and what fears they protect from.