Total immersion is the situation where the learner spends time in an environment operating mainly in French (the target language in this case). In this way the learner is completely surrounded by the language.
For a learner the immersion phase in the language learning process will really catapult your learning ahead, particularly if you communicate exclusively in the target language, and ditch your mother tongue.
One is then immersed in the language, and surrounded by the culture of the target language-speaking community.
Living with a host family will further bolster this learning process, and give you further insights into the mentality of the culture.
In reality it is very unlikely for people to really become fluent in a language without extensive periods of some form of total language immersion.