Labels refer to the categories of your data points, also referred to as a classification or class. You can think of them like targets, buckets, or types. In classification tasks, they all refer to the same thing.
Tip: Think of labels in terms of what you want to predict with your model once it has been trained.
Are you using Levity to determine whether a document is a resume or an invoice? Your labels will be resume, invoice, and any other document types you want your model to identify for you.