Marketing contacts is a newer feature in HubSpot that allows you to "set" a contact as someone that you market to -- or not. This not only affects your billing, but it does make it possible to keep hundreds of thousands of contacts in your CRM - but only pay for the ones you want to market to.

But you need to know how to set marketing contacts in HubSpot in order to take advantage of this feature. Note: This only applies for HubSpot marketing hub!