An MPP, or partner, means you may post behind a paywall and earn for your own writing, while also helping those your read earn. A little yellow star will appear with paywalled posts; this also indicates to possible reading writers that your part of the earnings circle. A FOM (Friend of Medium) is supposed to allow you to help the writers you support with 4x the earnings for full engagement.

My understanding is: As a $5/month partner, everyone earns pennies when you engage on their work, and vice versa. When a FOM reads your work, you earn 4x as much from their full engagement. Being a FOM doesn't take more earnings; it only bestows more earnings.