Three short lines, and almost nothing happens, which is part of the point. The poem gives us a single image, a white bear, and then a quiet shift in time: evening arrives, and "we get small."
The first line sits alone, as if the bear has just appeared in view. There's no description of what it's doing. It simply is, large and pale against its surroundings. Then evening comes, and the speaker says "we," pulling the reader in. The smallness isn't only physical. Judging by the setting the title suggests, it feels like a shrinking of human importance in front of a vast, cold landscape and a creature that belongs there.
The mood is hushed and humbling. There's a hint of unease, since a bear is powerful, but also a kind of calm acceptance. Nobody panics or boasts. The speaker just notices how the light fading and the animal's presence make people feel their true size.