he priest carries the sacred host, presumably to a man who's too ill to attend mass. Death goes in front, ringing a bell to ensure a speedy walk through the crowded streets, but no matter how fast they walk, Death will precede them.
The letters IHS are written at the top of the priest's ceborium. Some Christians believe this is short for Jesus, who in Greek is spelled IHSOUS. Other Christians believe IHS is an acronym for Iesus Hominum Salvator (=Jesus, saviour of men).
And then again there are Christians, who believe that the act of eating the flesh of ones God every week in church is an old pagan rite, and that IHS stands for the 3 "pagan" gods Isis, Horus and Seb.
It's interesting to compare the picture with one by Jost Amman from »Eygentliche Beschreibung Aller Stände auff Erden«. Amman's book is from 1568 - i.e. 30 year later than Holbein's, and Amman includes precisely the same elements that Holbein does: ceborium covered by cloth, lantern, bell and iron pot with holy water and sprinkler (aspergillum).
Variations: Birckmann alters the buildings in the horizon - e.g. he adds a round tower with a pinnacle at the top; The gate in the wall is opened, and two people are standing in the gate. Deuchar imitates Birckmann.