Before you can add concentrations, you must first add segment definitions to your organization. When we were adding audience data, we passed a demographic_id of 0 in our JSON request. 0 is defined in our system as the ALL segment, meaning everyone exposed to a unit. All other segment are user-definable and thus must be defined before they are used.
For example: you may have data that represents the concentration of people who play video games. You need to first define that video games is a segment that can be targeted before you can say "Hey! This screen over here at 7AM on Monday has a concentration of 0.04 for people that play video games".
Demographics define target audiences. In very general cases, this can mean that a demographic might represent a gender, or perhaps an age range. In a more specific case, a demographic might represent shoppers who have been observed inside of Walmart locations at least 3 times in the past 2 months.
Demographics are very specific to your needs, and are directly related to the audience concentrations that you will read about below.