Within an Account you may have many Contacts. Out of the box you have the ability to sort these Contacts by a field value (ex by name alphabetically) but sometimes more complex logic would be helpful in sorting Contacts so the most useful ones are up at the top.
This post shows you how to create a formula field that you can sort your related list by to accomplish this.
This logic will fluctuate based on your CRM and business processes. Data points you may want to consider are those that indicate that sales is currently reaching out to that Contact, the Contact has a desirable title/persona, and/or whether or not that Contact is no longer in the business.
Create a formula field which returns a number (I named mine "Order" but you can choose a different name if you like)
Using an IF statement, build out the logic so the Contacts you want at the top return a "1" and the rest of your Contacts are sorted by the appropriate priority.
Navigate to Account Page Layouts and edit the layout you would like to update.
Scroll down to Related Lists and edit the Contact related list.