You will find the "seat" terminology on the "Start an Organization"; you may also find this term in an email if you are asked to join an organization.
What a "seat" simply means is a license that is filled by a user.
So if you have an organization and you want 40 people to join; the number of open "seats" you will need is 40 seats. You can think of it as a conference room with a certain amount of chairs for people you invite to sit in.
Increasing the amount of seats increases the amount per month you are charged if you are the organization or enterprise account owner. People that you invite to your organization will not be charged.