An Authorization Rule allows configuring a custom authorization / approval on a documents, based on conditions defined.
Example: If a Sales Order's Grand Total exceeds $1,000, then it should be verified/submitted by the Sales Manager only, even if the Sales User has "Submit" permission.
On the same lines, you can define Authorization Rule based on the fields like Net Total, Grand Total, Discount % and specify who would be the document approver if the authorization condition is matched.
Let's consider a detailed example of an Authorization Rule to learn better.
Assume that the Sales Manager needs to authorize Sales Orders, only if it's Grand Total value exceeds 10,000. If the Sales Order value is less than 10,000, then even the Sales User will be able to submit it. This means that the Submit permission of Sales User will be restricted only up to Sales Order of Grand Total less than 10,000.
If the Sales User tries submitting the Sales Order of value higher than 10,000, then he will get an error message.
If you wish to restrict Sales User from submitting Sales Orders, then instead of creating Authorization Rule, you should remove submit privilege for Sales User from Role Permission Manager.