This page was added in 2.1.0. It lists all available placeholders that can be used in Form Tools fields, specifically the Default Values for New Submissions setting in Edit View » main tab. These placeholders, when entered in that field, will have their values automatically inserted for all new submissions created by that client account when they create a new submission via the Form Tools UI. This lets you do all sorts of things like tracking who creates submissions, to configuring client accounts to only ever be able to edit their own form data and no other clients assigned to the form.
If you don't see your field listed here, edit the field and add the Field Identifier value. It will then get listed and show you the placeholder you can copy and paste into the Core field.