What do windows and doors have in common (besides often being rectangular)? Yup, they are places where we can enter or exit to another side. Housman uses lots of door and window imagery to emphasize the sense that death is like moving from one side to another. The speaker seems to want to make death seem a little less scary, a little more familiar, by making it seem like passing through a doorway of sorts.
Line 7: The speaker tells us that the pallbearers carrying the athlete's coffin set it down "at [the athlete's] threshold." A threshold, as many Shmoopers probably know, is another (kind of poet-y) word for a doorway or entrance. We can imagine the mourners setting the coffin down at the mouth of an open grave (a kind of a doorway in the ground and metaphorically a doorway into the afterlife) or at the doorway of a tomb of some kind.
Lines 21-24: Stanza six picks up the threshold imagery that was introduced in stanza two. This time, the coffin is "on the sill of shade." A "sill" is that ledge below a window but it's also a name for the bottom part of a doorframe. So, the athlete is placed at the doorway of "shade." Shade is nice on a hot day, but the shade in this line is a metaphor for the land of the dead—a pretty chilly place.
And just in case we missed any of the previous doorway imagery, Housman hits us with one more reference in line 23. The speaker tells the mourners to hold the athlete's trophy (presumably the one he got for winning that big race) up "to the low lintel." A lintel is a tasty kind of legume often used in Italian and Mediterranean cuisine… no, wait. That's a lentil. Sorry. We're getting hungry.
A "lintel" is actually the horizontal beam above a doorway. So, in stanza six, Housman references the doorway's sill (at the bottom) and the lintel (at the top). That's a lot of doorway detail for one stanza. Housman really wants us to consider this athlete's death as a passing through rather than a passing away.