Justified by Faith - Romans 4:4-5

What does it mean to be justified by faith? What role do our works have in justification? If not our works, then who can justify us?

Romans 4:4-5 KJV

4 Now to him that worketh is the reward not reckoned of grace, but of debt.

5 But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness.