"All Summer in a Day" is set on Venus, therein described as a planet whose rainfall lasts years at a time with only momentary breaks in the deluge. This story concerns one of those brief moments of sunshine, where the children are allowed to leave the underground tunnels in which the population otherwise resides.
With this in mind, to put yourself in Bradbury's shoes: how would you set about describing an alien world? First, note that Bradbury specifically labels the planet's surface as a jungle, but the task remains: how do you set about creating an impression of a place that does not exist on Earth? The selection quoted serves as part of Bradbury's description, as he attempts to, through use of imagery, create a vivid visual picture of Venus's jungle surface.
Note, in particular, his use of metaphor as he compares the strange jungle flora with octopi, a metaphor that is further extended in the description that follows (a description in which Bradbury effectively blurs the lines between flora and fauna to such a degree that any technical distinction between them tends disappear beneath the rhetorical flourish). At the same time, however, also note that the passage actually continues after the point at which your quote ends, with Bradbury further writing,
It was the color of rubber and ash, this jungle, from many years without sun. It was the color of stones and white cheeses and ink, and it was the color of the moon.
The purpose, for Bradbury, is to establish in his readers' imaginations a vivid visual impression of this alien jungle, one that is entirely unlike any jungle that can be found on Earth.