PBMs determine which drugs are covered by insurance plans, set prices for drugs, determine which drugs are considered tiered (in other words, you must try this drug before you try this drug), so forth.
You're welcome. In theory, they work vaguely similarly to how pharmacy benefits work under countries with single-payer systems. Effectively, because thousands/millions of people are enrolled through a single entity, a drug company gives that group of people a "volume discount" of sorts based on pricing pressure.
PBM: "Hey, Merck. We have a million people."
Merck: "OK, we'll offer you this drug for this price instead of this price."
You get the idea.
Consensus cracking intensifies.