OpenID realm is mostly used when you are migrating from OpenID 2.0 protocol to OpenID Connect.
Basically if don't know what to specify there you may leave it blank, but in general this field value must correspond to openid.realm
request parameter (optional) when building an authentication request to Google.
Please, read Migrating from OpenID 2.0 to OpenID Connect and OpenID Connect documents from Google Identity Platform section for more details.
P.S. please, tag you OpenID related question with google-oauth
label to get the response quickly ;)