a large jar or skin for storing liquids, especially a wine container
This is the everyday storage vessel of ancient Israel: a big container, made either of animal hide or of fired clay, used to hold and carry wine, water, and other liquids. It is the kind of thing you would load onto a donkey for a journey or bring to the sanctuary with an offering.
Because the word names the container by its function rather than its material, English translations reach for several different words for it, from wineskin to jar to pitcher to flagon. The same vessel could be a supple leather bag or a heavy earthenware jar, so the right English word depends on the scene.
Poets could also stretch the picture upward: the clouds become the waterskins of heaven, tipped over to pour out rain. The vessel is fragile too, and prophets use its easy shattering as an image of ruin.