Bitachon is put simply: the sure knowledge in your heart that HaShem loves you and will care for you like a father for a son, regardless of your past, present or future circumstances.
Bitachon overrules all possible objections raised by the "other" side, allowing for HaShem to provide for you things you don't even deserve.
Sounds pretty fundamental, so why wasn't it mentioned more prominently in the Torah? Actually, the Netivot Shalom explains, it was. The second half of the very first of the Ten Commandments only makes sense when you realize it is a Commandment to have Bitachon. Bitachon is, as the Rebbe explains, the other half of Emunah. God commands us to believe that He is our God. Anochi HaShem Elokecha. What comes next? Asher Hotzeitichah M'Eretz Mitzrayim -Who took you out of Egypt. Why mention Egypt? Why not mention that He created the Heavens and the Earth? The answer of course: Bitachon. How So? Remember that HaShem loves you so much that He called you His first-born son and rescued you through many wonders and miracles from Egypt, while you were mired in the 49 gates of impurity. No matter how low we sink, HaShem still loves us more than we can imagine. The absolute knowledge of this is Bitachon, the deeper and hidden level of Emunah, collectively the first commandment.