A sales-qualified lead is someone who has been identified by a salesperson as a prospect for the company. A sales-qualified lead is not necessarily the most valuable client and can be considered someone who may get a call from the salesperson later on.
Sales-qualified should be treated differently than other prospects. They are not necessarily the best possible client, but they are still important to follow up with. After all, they are potential customers, so it makes sense to follow up with them and ensure they are not wasting their time on other prospects.