In the poem I just read, scrim is probably used to mean "curtain." I don't know what I would have said it meant before I looked it up. Maybe some type of border or fabric-related item? Hmm. Well, as one may see in the definition below, scrim had a very specific meaning which has broadened over time.
Scrim Scrim, n.
1. A kind of light cotton or linen fabric, often woven in
openwork patterns, -- used for curtains, etc,; -- called
also India scrim.
2. pl. Thin canvas glued on the inside of panels to prevent
shrinking, checking, etc.
-- From The Collaborative International Dictionary of English v.0.48
n : a firm open-weave fabric used for a curtain in the theater
-- From WordNet (r) 2.0
34 Moby Thesaurus words for "scrim":
act drop, asbestos, asbestos board, backdrop, batten, border,
cloth, coulisse, counterweight, curtain, curtain board, cyclorama,
decor, drop, drop curtain, fire curtain, flat, flipper, hanging,
rag, scene, scenery, screen, side scene, stage screw, tab, tableau,
teaser, tormentor, transformation, transformation scene, wing,
-- From Moby Thesaurus II by Grady Ward, 1.0