What is meant by the quote And hold to the low lintel up?

a lintel is the horizontal support above a doorway, so to hold it up would keep the wall from collapsing and it would keep the doorway open.

"Hold to the low lintel up, the still defended Challenge cup", from A. E. Housman's "To an Athlete Dying Young".

In this usage, the "low lintel" is the crossbeam above the doorway to the athlete's tomb and the "Challenge cup" won by the deceased athlete is being held up before the tomb entrance.