What is the interpretation of the Almighty's saying: "...and our extravagance in our matter"?

What you asked about is part of the prayer of the sincere believers in the Quran: "Our Lord, forgive us our sins and our excesses in our affairs, and plant firmly our feet, and give us victory over the disbelieving people." Al-Qurtubi interpreted "our sins" as minor sins and "our excesses" as major sins. Excess (isrāf) is extravagance and overstepping bounds. The Prophet, peace and blessings be upon him, also supplicated with this prayer: "O Allah, forgive me my faults, my ignorance, my excesses in my affairs, and what You know better than I do." Therefore, a Muslim should use the supplications mentioned in the Quran and Sunnah.