Who is a Sales Qualified Lead?
A sales qualified lead is referred to a prospective customer that has been researched and vetted by the marketing department, then qualified by the sales department and is ready for the sales team to get in contact for further engagement.
Initially you capture Marketing Qualified Leads(MQLs) who have reached out to you by organic means of any other marketing efforts. Once you have captured a lead who has expressed interest in your products to some degree, the lead is further researched and vetted by sales and marketing teams to gauge the lead's sales readiness.
Sales and marketing teams work together to determine which actions qualify a prospect as ready to move on to the next sales process. After all the criteria of qualification is passed, the MQL becomes SQL and handed over to the sales rep for further engagement to close the deal.
Defining a set of actions for qualification is important, otherwise your marketing team might pitch leads that aren't ready to move on to the sales process and it'll slow the productivity. For example, if someone is reading your blog regularly, downloaded your e-book and subscribed to your newsletter - your marketing team might consider this person as a prospective client, but there could a possibility that a lot of persons are performing the mentioned actions just for knowledge and are not fit for your product(like students, individual readers).
Generally, for a lead to become an SQL, they have to have a need for your product or service, have the budget to purchase your product and infrastructure to use it, and your product or service solves their pain points.