"MQL" stands for Marketing Qualified Lead. A person is considered an MQL when they start to engage with your business. Engagements could include multiple website visits, content download(s), or filling out a form to request information.
MQLs are more likely to become customers than your other leads because they are showing interest in what your company has to offer by interacting with your content. However, MQLs are not guaranteed to become customers. MQLs are interested in learning more about what you have to offer but are not ready to "pull the trigger" and buy quite yet.
When an MQL is ready to make a purchase they transition to the lifecycle stage known as an SQL.