First, a simile is a literary term in which the words "as," "like" or "than" are used to indirectly compare two different things. Similes differ from metaphors in that their comparisons remain implied, while metaphorical analogies are more direct.
Author Khaled Hosseini uses the simile several times in Amir's narrative of the description of Baba in Chapter Three. When the huge Baba entertained at parties,
... attention shifted to him like sunflowers turning to the sun. (The people's attention is compared with sunflowers.)
Baba drew attention even while asleep.
... the sounds of Baba's snoring--so much like a growling truck engine--penetrated the walls. (Baba's snoring is compared with a truck engine.)
Later in the chapter, Amir describes the action of the Buzkashi match when one of the chapandaz fell from his horse and was trampled.
His body was tossed and hurled in the stampede like a rag doll... (The man's body is compared with a rag doll.)
Unhappy with his son's lack of masculine pursuits, Baba claimed that Amir was
"... always buried in those books or shuffling around the house like he's lost in some dream." (Amir's reading and shuffling are compared with dreaming.)
I hope some more example of similes are of use. In Chapter 5 of the book, right after Amir has read his very first story to Hassan, a coup occurs in Afghanistan. The chapter opens with a simile:
Something roared like thunder (35).
This is a simile because whatever roared, it roared "like" thunder.
Another simile in Chapter 5 describes Assef, who is Amir's nemesis throughout the novel:
He walked like a Khan strolling through his land with his eager-to-please entourage (38).
A Khan would be the Afghan equivalent to a king, so Assef is strolling around like a king.
A third simile in this chapter occurs near the end, when Hassan is presented with plastic surgery as his birthday gift. This causes Amir to recall the occasion of his circumcision, which he was promised would not be painful, but which he describes with a simile this way:
But when the numbing medicine wore off later that night, it felt like someone had pressed a red hot coal to my loins (46).
Try to find some similes, too. There are many more in this story. Look for the word "like" or "as" to help you find them.