In most CRM systems, the term sales qualified lead (SQL) designates a lead that has been vetted to fully fit prospective customer profile. The SQL pool is formed by working with marketing qualified leads (MQL) and weeding out leads that don't fit the SQL profile. Here's an example that shows the difference between SQL and MQL.
Suppose you sell X-ray machines to dental clinics. Your experience shows you in order to sell you equipment successfully, the following conditions must be true - the clinic should have at least five dentists working there and their existing X-ray unit has to be at least five years old.
Your marketing department obtains a list of all dental clinics in the state. These are MQLs, because dentists are your target market. However, this list contains no information about the number of dentists working for each clinic and the age of their X-ray equipment. You'll need to call or visit each clinic in order to obtain this information. Only the clinics that meet your buyer profile will be added as SQL. Most modern CRM software allow you to automate the process.