Is the Hadith: "Indeed, these graves are filled with darkness for their occupants, and Allah illuminates them for them by my prayer" authentic, what does it mean, and how does it apply to us?

The hadith is authentic, and its meaning is that graves are filled with darkness, illuminated only by righteous deeds or accepted intercessions. Among the means of attaining this light are maintaining prayer, patience, performing Hajj and Umrah, participating in circles for studying the Qur'an and Islamic knowledge, and the supplication of those who pray over the deceased.