Symbol Analysis
A hill is not a tent—actually, they're two extremely different things. A hill is solid, made of dirt and stones all piled up (we're not geology experts, but we gather that's the idea). A tent isn't really solid, though. It's, like… some fabric. That difference helps to set up the radical nature of the metaphor put forward at the end of stanza 3:
The tent of the hills drummed and strained its guyrope,
The fields quivering, the skyline a grimace,
At any second to bang and vanish with a flap; (12-14)
The wind makes the hills like a tent, because it seems to take away their solidity and make them as transitory as a tent. They can be upended, blown away, snapped up—just like that.
These tent-hills, then, remind us that the whole world seems totally fragile when faced with the awesome power of the wind. All the physical things that exist in this natural landscape are subject to a power that might destroy them at any moment. And that power is what the wind represents.
P.S. What's a guyrope, you ask? It's a rope that helps give a structure support, like the ropes that help hold up a tent. (You're welcome.)