Metadata is data that provides information about your files in Prostream.
Metadata is data that describes the characteristics of certain data. So, it actually data about data. The metadata for a certain document is, for example, the author, the date of writing, the number of pages and the language in which the data is written. Explicitly adding metadata in Prostream has the advantage that you'll be able to filter and find your documents more easily.
Metadata gives your documents valuable context. When you use sets, you ensure that all documents in a folder are provided with extra properties (items) that are relevant to your projects.
Meta data sets are a composition of metadata items in a fixed order. You can use items in multiple sets and make each set mandatory.
Sets can be assigned to a folder, so all documents in that folder are provided with the fields in your set, and give extra context to your documents.
By default, a metadata set contains the metadata items document name, uploaded on, uploaded by and version number. You are obliged to give the set a name (title) and you can add extra items.
How to create a new metadata data set can be read here.