For those who don’t know, Castle Black was one of the main locations on the Wall where members of The Night’s Watch – the sworn brotherhood of men who guard the North against the Wilding and Night King.

Source: https://www.koimoi.com/television/game-of-thrones-castle-black-didnt-just-look-foul-it-was-stinking-with-pigs-raw-meat-heres-how-jon-snow-kit-harrington-others-lived-there/

When he arrives at Castle Black, the Night's Watch doesn't even have a Lord Commander, as the Wildling crisis is at its height after Jeor Mormont's () death, so Ser Alliser Thorne () is acting Lord Commander.

Source: https://collider.com/game-of-thrones-book-change-jon-snow-death/