Abram is afraid and doubting: he has no son and his heir will be a servant. God takes him outside at night, shows him the stars, and promises his descendants will be as many as the stars. Abram believes God, and God counts this as righteousness. God then makes a formal covenant — a solemn promise — by passing between cut animal halves as a smoking firepot and burning torch, binding Himself to fulfil the promise of the land to Abram's children.
God made a real, unbreakable promise to Abram — and Abram trusted God, even when he couldn't see how it would happen. That trust is called faith.
Genesis 15:6 is the most theologically important verse in the chapter — Paul quotes it in Romans 4 and Galatians 3 to show that justification has always been by faith, not works. The covenant ceremony (v9-21) is an ancient Near Eastern practice where both parties walk between slaughtered animals ('may it be done to me as to these animals if I break this covenant'). Only God (the fire) passes through — the covenant is entirely His initiative and responsibility. For children, the key image is God coming to Abram personally and making a promise He would keep.
We can trust God's promises even when we can't see how they will come true. Abram looked at the stars and believed — we can too.