What is the meaning of Abdullah ibn Mas'ud's saying: "Count your evil deeds, for I guarantee that Allah will not let your good deeds go to waste"?

The mentioned sentence is part of a long narration from Ibn Mas'ud (may Allah be pleased with him) found in Sunan al-Darimi and Musannaf Ibn Abi Shaybah, and authenticated by Al-Albani.

Its meaning is: that a Muslim should beware of sins, innovations (bid'ah), and newly invented matters, and hold himself accountable for them, and increase in good deeds which will not be lost with Allah.

The narration came in the context of Ibn Mas'ud's (may Allah be pleased with him) warning against an innovative method of collective remembrance of Allah (dhikr). He saw people sitting in circles remembering Allah with pebbles, guided by a man in each circle. Ibn Mas'ud then stated that if they had counted their sins, he would guarantee that none of their good deeds would be lost, disapproving of their action and warning against the rapid destruction of the Ummah through innovations. He reminded them that the Companions were still numerous and that the clothes of the Prophet (peace be upon him) and his utensils had not yet worn out. It later became clear that the majority of these circles were the Khawarij who attacked the Companions on the Day of Nahrawan.

The narration warns against innovations and newly invented matters, and emphasizes that the essence of worship lies in following the Sunnah, and that a minor innovation can lead to a major one.