Dataset descriptions are great ways to let users know a bit about what the dataset is and how it might be used. Descriptions, as their name suggests, provide a human-readable way to understand the contents of the data, how it is aggregated, and how it should be used.
Descriptions can be anything you choose. Whatever you end up adding to the description is indexed by the catalog search, which means that the more context you add to a description, the easier the dataset will be to find.
To add a description to a dataset, open the dataset edit popover and write (or copy+paste) a description into the description field.