The act of "boiling water" can refer to two things.
1. Heating up an amount of water with the intent to make it reach a temperature of 100 degrees Celsius, which is the accepted standard boiling point of water.
2. A euphemism for smoking weed (or another substance) through a bong, since the sound the water makes during inhalation resembles the sound of water that has reached it's boiling point.
1. Max: Yo, I'm so hungry... gotta any food in the house Blake?
Blake: Yeah man, I've got some ravioli, let me just boil some water and it'll be ready pretty soon.
2. Blake: So what do you wanna do while the food is getting ready?
Max: Let's boil water! *makes gesture of lighting bong*
Blake: Hell yeah. *gets bong from behind couch*