A simile is a figure of speech that uses the words "like" or "as" to form a comparison between two unlike objects.
When Ralph, Jack, and Simon start to explore the territory around them to see if they are on an island, they find that they had difficulty, not with the steep ascent but with the undergrowth.
"Here the roots and stems of creepers were in such tangles that the boys had to thread through them like pliant needles." (pg 26 - chapter one)
This compares the roots and stems of creepers -- a form of crawling underbrush -- with pliant needles. The needles suggest that these roots and stems poke into them as they walk. The word "pliant" means that they yielded to the boys movement.
Another simile is when the boys gather wood to build a signal fire for the first time. They start the fire and gather around it.
"The flames, as though they were a kind of wild life, crept as a jaguar creeps on its belly toward a line of birch-like saplings that fledged an outcrop of the pink rock." ( pg 44 -- chapter two)
This simile compares the movement of flames with a creeping jaguar. These are two unlike objects and the image here is a slow, low lying flame moving toward some other trees.
The page numbers I have given are for my edition of the book. I have included the chapters so that they may be easily found. They should be close to that page though.