The provider is nothing but the membership account that you use. This is apple developer account.

In the developer account there can be different membership account inside that and that account is the provider for the certificates or profiles or identifiers you create

Provider short name can be known from the membership account page for reference see the image below

altool manual page

The manual for altool can be viewed by running the command man altool. The manual states:

--asc-provider provider_shortname

Required with --notarize-app and --notarization-history when a user account is associated with multiple providers.

Without your knowing, it is unlikely that your account is associated with multiple providers.

asc-provider

If you believe you need a provider, see Apple's Customizing the Notarization Workflow:

You can omit the asc-provider option for the notarization call if your credentials are associated with only a single team. Otherwise, use the option to indicate the provider short name for the appropriate team. Starting in Xcode 11, you can use altool to get a table of providers, provider short names, and team IDs associated with a given username and password using the list-providers option:

xcrun altool --list-providers -u "AC_USERNAME" -p "@keychain:AC_PASSWORD"