What does the phrase "it's as if they had slipped under a giant colander" mean in chapter 9 of Tuck Everlasting?

A colander is a kitchen tool. Shaped like a bowl, it has many holes in it. Its purpose is to drain things like cooked spaghetti noodles or rinsing fresh fruits and vegetables. If you were under said colander as it was draining something, you'd be getting wet. Apparently the characters were under a tree or bush that was shedding water in small streams. This shelter would be better than standing in the pouring rain but not much better. This is an example of a simile because it uses the word "as".

How is the following excerpt from Tuck Everlasting a simile: "and it was if they had slipped in under a giant colander"?

This is a simile because the author makes a comparison between  two unlike things using the words “like” or “as.” In this case, the author is comparing the feeling of being under the treetops to the feeling of being under a giant, overturned colander. 

A colander is a bowl with small holes in it. It’s usually used to drain pasta or lettuce. You probably have one in your kitchen! If you can imagine the terrifying situation of being trapped under a giant colander, you can see why the comparision actually works. The small holes would only let in tiny pinpricks of light, resulting in an effect that some might call “dappled light.” Likewise, the trees over Winnie’s head are so thick and old that they only let in little tiny dots of light.

This simile creates a romantic, mysterious atmosphere.