If you have a SAML Metadata file/URL from your IDP, you can configure the plugin by uploading the metadata file or configuring the metadata URL.
The Select IDP drop down will have some IDPs like ADFS or Azure AD. The Metadata URL for these IDPs will have to be generated using some IDP related data that you will have. For example, if you select ADFS you will be asked for the host-name of your AD server. The metadata URL will then be generated based on the host-name that you enter, and the plugin will retrieve IDP metadata rom this generated URL. There are also IDPs like Okta, G Suite or One Login where the process for getting metadata is standard. For example if you choose G-Suite, you will by-default be asked to upload a metadata file.
For IDPs other than the ones mentioned by name in the drop-down, you may choose whether to upload a metadata file or to give a metadata URL. The options to do so can be found in the same drop-down.