Bittachon expresses a vote of confidence in the will of God. We rely upon God's emunah (faithfulness) and character as the only true source of our hope. As the Psalms repeatedly state, the confidence that comes from relying upon God is valid only because of God's chesed (his loyalty or covenantal faithfulness), not because of our own merit or faithfulness.
Some of the Jewish sages have said that bittachon can be of two forms: "active" bittachon and "passive" bittachon. Active bittachon is self-confidence to carry out actions and goals we set for ourselves. However, it is by God's grace that we are enabled to do anything at all, and this recognition leads to passive bittachon, namely, the confidence that God will work all things together for good to those who are trusting in Him (Romans 8:28). The one who trusts in the Lord is blessed, since his expectation shall never be dashed.