A webmail provider is a service that hands out free mailboxes to anyone who asks — Gmail, Outlook.com, Yahoo Mail, iCloud Mail, GMX, Proton and thousands of smaller ones — as opposed to a company running mail for its own domain. Most consumer addresses live at one, which is why a signup form that refuses them refuses most people.
What Spaw answers
free_provider reports whether the domain is on the open free-provider list, refreshed weekly, and domain_category answers webmail. Two checks are skipped on purpose for these domains: the registration age, because the provider's domain is decades old and says nothing about the person, and the no_spf weight, because the provider's authentication posture is known. The mailbox is still probed, and the provider's own username rules are applied before the handshake so an impossible username never spends one.
What it does not mean
A webmail address is not a lower-quality address, and it is not disposable: those are separate lists with separate weights. It is an address whose domain says nothing about the owner, so every remaining signal comes from the username and the handshake.