A data source is a table of reference data — a list of outlets, a product catalogue, a customer list — that a form can look up from, so a field worker picks from a real list instead of typing free text.
This is different from a submission: a data source is data you bring into the system to help build forms; a submission is data field workers capture using a form.
Go to Data sources → New, upload or connect your data, and map its columns. Once saved, any form field can be configured to look values up from it (e.g. an "Outlet" dropdown populated from your outlet list).
If the underlying data changes (new outlets added, prices updated), use Refresh/Sync on the data source's detail page rather than recreating it — forms referencing it keep working without needing to be republished.