What is meant by the practical realization of Tawhid (monotheism) and how is that achieved, and does repentance from sins - no matter how great - save one from the torment of the grave and Hellfire?

Achieving Tawhid (monotheism) and sincere repentance are among the means of salvation from Allah's punishment in this world and the Hereafter. Achieving Tawhid means purifying it from the impurities of shirk (polytheism), bid'ah (innovation), and sins. This is accomplished through three matters: knowledge, belief, and submission. If Tawhid is achieved, Paradise is guaranteed. As for sincere repentance, with its conditions, it erases sins and removes punishments.