A simile is a comparison that uses the words "like" or "as." Similes are commonly used by writers to create vivid pictures in readers' minds.
For example, the rabbits sitting on the sand banks in chapter one, just prior to George and Lennie's arrival, are described as sitting "as quietly as little gray, sculptured stones." The image of stones, which do not move, creates a clear picture of just how unmoving the rabbits are.
Afterwards, when Lennie is drinking from the green pool that he and George come across, he is described to be "snorting into the water like a horse." This simile uses the word "like" to create a clear picture of how messily and eagerly Lennie is drinking.
Another simile using "like" is used later in the first chapter to describe a water snake moving on the pool. The snake's head is compared to "a little periscope," which creates a clear image of the snake's head protruding out of the water.
Later, Lennie begins to cry after George throws away the dead mouse that he has been carrying around in his pocket. His crying, or "blubberin'," is described by George as being "like a baby." This commonly-used simile gives the reader an indication of how distraught Lennie is.
When George and Lennie are at the ranch and see Curley's wife for the first time, another simile is used to add to the picture of what she looks like. The "rolled clusters" that her hair is hung in are compared to sausages using the word "like".
The last simile that I'll mention is found in the first paragraph of chapter five. The hay in the great barn is described as being "like a mountain slope." This image creates a clear mental picture of the volume of hay and the shape of the pile of hay.