I can't understand the meaning of the phrase in a book

This is taken (although you didn't say) from a novel called "It's Kind of a Funny Story" by Ned Vizzini, which is used in a number of ESL courses.

There’s always pot at Aaron’s house; he has a room with an entirely separate ventilation system and lockable door that his parents could rent out as another apartment. Resin streaks outline his light switch, and his bedsheet is pockmarked with black circles. There are stains on there, too, shimmery stains which indicate certain activities that take place between Aaron and his girlfriend. I look at them (the stains, then the couple). I’m jealous. But then again, I’m beyond jealous.

Pot is another name for marijuana or cannabis (a drug) which is consumed by smoking, as dried plants, or as resin extracted from plants. The resin is sticky and can stain things, especially if the user is not inclined to be tidy or clean (this is clear from the text). There are streaks of resin (from people's fingers) around the light switch (on the wall) forming an outline.