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".