The context is like a "magic" argument that the web client will always give to the server when calling a method. The context is a dictionary containing multiple keys. One of the most important key is the language of the user, used by the server to translate all the messages of the application. Another one is the time zone of the user, used to compute correctly dates and times if Odoo is used by people in different countries.

model.call("my_method", {context: new instance.web.CompoundContext({'new_key': 'key_value'})})
@api.model
def my_method(self):
    print self.env.context
    // will print: {'lang': 'en_US', 'new_key': 'key_value', 'tz': 'Europe/Brussels', 'uid': 1}

Note-You can see the dictionary in the argument context contains some keys that are related to the configuration of the current user in Odoo plus the new_key key that was added when instantiating CompoundContext().