If the webs aren't anchored between two solid masses (such as walls or trees) OR layered across a floor, wall, or ceiling, the conjured web collapses on itself, and the spell ends at the start of your next turn. Webs layered over a flat surface have a depth of 5 feet.
I'm having difficulty imagining webs not attached to something. Are they stacked up like pancakes? Does this mean I can just throw webs on the ground and not worry about attaching them to something? What are the disadvantages/advantages of anchoring/not anchoring? Or should the "or" in web be an and?
The question "How exactly does the Web spell work?" is similar, but I'm asking about D&D 5th edition, not Pathfinder. The wording is very different.