Welcome to the BrandIndex API!
An API ("Application Program Interface") is a group of things that makes it easier to use a certain computer program (or set of programs): a programming specification, a protocol with which a API user can access a certain program (or set of programs), and the underlying program(s) that provide data and/or behavior. In other words, it's a programming interface through which another program can access the software application that is provided by this API.
One of the basic meanings of this is that an "API" is not something made for human users to use directly. It's not a user interface (UI) with buttons that the user can press and make something happen; rather than that, an API intends to be a software layer provided by an application which another application (normally running in another computer) will use. The reason for this is so that the API doesn't make any assumptions about the UI that is going to be used for accessing this application, hence leaving the UI decisions to however implements a piece of software that "consumes" this API.
Now let's translate this explanation into an example tangible to BrandIndex; the way we provide our product called "BrandIndex API" is through a webservice - that is, the application that contains the data which the user will be interested in lives in another computer (which we call "server" - because it serves the application for other computers to use). When another program, running in another computer, connects to this API server, in order to use its data and behavior, we call this other program "API client" (or, in the context of webservices, a "webservice consumer"). Just be double-clear, then: an "API client" is another computer program which uses the "API server".
As to the BrandIndex API itself, it was made to provide data that is already available through the BrandIndex UI, but, instead of targeted to humans, targeted to API clients which other developers will develop in order to display the data in whatever way they want or need. For example, if another company wants to access our data in order to display another kind of chart (composed by this data) in their own website, they can create an API client to be able to retrieve the data.
The data that we provide through the BrandIndex API, then, is provided under different formats, but as close as possible to the already-processed data that we use to generate the charts - rather than providing raw data (respondent-level data). This way, we can send along the scores, volumes and other pieces of information that we generate as a result from aggregating the data, matching the numbers displayed in the BrandIndex UI charts.
Good luck using the API!