When a field on a record's form, or in a table's list view, is derived from another table by dot-walking through a reference field, that's called a derived field. For example, if you have a field called Requestor on a task record, which is a reference to the sys_user table, you could also have a field called Requestor Email, which would contain the value of the email field from whatever user record is referenced in the requestor field. Assuming the obvious field names for Requestor and Email, the field name for this derived field would be requestor.email (It uses dot-walking!)

What's important to understand about derived fields, is that if you don't explicitly protect them with an ACL, UI Policy, or otherwise, users may be able to modify these fields, which actually modifies them on the source record! This means that modifying the Requestor Email field on this Task, for example, would result in the Email field of the sys_user record being modified, since the Requestor email field is not actually a field on the record you're viewing!