The seven triangles mean to recycle, but it gets confusing for people to know what type of materials go in which bin. The blue bin is for recycling, the green bin is for compost and the black bin is for trash.
If people do no separate their garbage properly, "contamination" (or mixing of materials) happens. This is a huge problem according to SF Recylcling.
Here is SF Recycle's page about recycling materials for the blue bin (it's trickier than you think — coat hangers cannot be recycled).
This triangle collects soda cans, water and salad dressing bottles, and peanut butter and jam jars.
This triangle collects water pipes, rings, milk, water and juice bottles, and shampoo bottles.
This triangle collects juice bottles, cling films, and pvc piping.
This triangle collects frozen food bags, squeezable bottles, honey, mustard, cling films, and flexible containers.
This triangle collects microwwaveable ware, kitchenware, yogurt containers, margarine tubs, and disposable cups and plates.
This triangle collects egg cartons, packing peanuts, disposable cups and plates, trays and disposable containers.
This triangle collects beverage bottles, baby bottles, and electronic casing.
Here is a Wordle highlighting key words about recycling plastic:
Image and research information from Wikipedia.