a covering: an outer layer laid over something to protect or enclose it
Nearly every time this word appears it names the outer covering of the tabernacle: the layers of ram skins and fine leather spread over the sacred tent to shield it from sun and weather (Exodus 26:14; 40:19). It is the protective top layer, the part a person would see looking down on the tent from outside.
In the book of Numbers the same word takes on a portable sense. When the Levites broke camp, they wrapped each holy object in a covering of leather before lifting it onto the carrying poles, so here the covering travels with the furniture rather than staying fixed over the tent (Numbers 4:8-12).
The one use outside the sanctuary keeps the same idea: Noah "removed the covering of the ark" to look out over the drying land (Genesis 8:13). Whether over a tent, a wrapped object, or a boat, the word points to a protective layer laid on from above. It sits close to a covering that hangs across a doorway (מָסָךְ) and to a covering worn as a garment (מְכַסֶּה), but those are draped or worn, while this one is laid over the top.