What does Daniel 9:24 mean?

'Seventy sets of seven have been set for your people and your holy city to finish rebellion, put an end to sin, make things right, bring in lasting goodness, confirm what the prophets saw, and set apart the most holy place.' Daniel 9:24 - Modern Text Bible

(Gabriel reveals a period of 'seventy sevens' (often understood as seventy weeks or seventy sets of seven years) that will bring about the fulfillment of God's promises and the end of sin.)