The SQL stage is where sales takes ownership. A rep has looked at the contact, confirmed there is a plausible need and fit, and committed to working it. HubSpot records the date the contact entered the stage, so you can measure how quickly MQLs are reviewed and how many are accepted.
Who sets the stage matters. MQL is usually set by marketing automation, but SQL should reflect a sales decision. Common patterns are a rep setting it by hand after a first conversation, or a workflow setting it when a rep changes lead status to connected or books a meeting. Either way, the rule should be written down so every rep applies it the same way.
The next step after SQL is usually a deal. When a deal is created and associated with the contact, the contact typically moves to opportunity. If SQLs pile up with no deals, either reps are qualifying too loosely or they are not creating deals in HubSpot, and pipeline reports will understate what sales is working on.
Measure the stage from both sides. Track the share of MQLs that become SQLs, the time it takes, and the share of SQLs that become opportunities. A sharp drop at any point tells you which side of the handoff needs work.