Vector data can have other data associated with it. This data is a simple database (.dbf), similar to and editable in an excel spreadsheet, arranged in rows (records) and columns (fields).
This database can be used to perform simple or complex queries and, crucially, to colour i.e. style your maps to help you create useful keys to explore and present your data.
When you first add a layer from a third party data set, you will have to link to the database in order to style using the database. You can always link to the database later, and there are 2 different options for linking to a database.
When styling, you will need to point to an appropriate field. When using third party data, make sure you’ve examined the database carefully before you start linking to it.
Inappropriate linking can slow Map Maker operations down considerably or cause it to hang / crash. Usually only a handful of fields are usually suitable for rendering text based keys or labels, while range-based keys can only use numerical or integer fields.