What is the explanation of the Hadith: "The first of people's deeds for which they will be held accountable on the Day of Resurrection is prayer," and do the regular Sunnah prayers compensate for deficiencies in the obligatory prayers?

The first thing a servant will be held accountable for on the Day of Judgment, concerning the rights of Allah Almighty, is prayer (Salat). If it is sound, he will have succeeded and prospered. If it is corrupt, he will have failed and lost. If anything is found lacking from its obligatory form, the angels will be told: "Look, does My servant have any voluntary (worship) by which what was missing from the obligatory prayer can be completed?" This includes deficiencies in its quality or abandoning prayer entirely. The regular Sunnah prayers are considered among the best supererogatory acts that complete the deficiencies of the obligatory prayers. This also applies to all other deeds, such as fasting, Zakat, and Hajj.