HubSpot uses this term regularly and you'll see it come up frequently in other places, the MQL. What it is exactly and what the difference is with, for example, an SQL, is explained below.
An MQL, or Marketing Qualified Lead, is a lead that matches your buyer persona criteria and is therefore interesting for your company. However, this person is not yet at a point of action to buy and therefore they need to be "warmed up" first - this is known as lead nurturing. This can be done by offering good content, such as blogs and videos, at the right time.
The MQL generally falls within the awareness or consideration stage, which means that they have become aware of the problem/challenge they face and are looking for a viable solution. This is where you can capitalize on the opportunity.
If a lead becomes even more enthusiastic and starts to interact more with your content, then we speak of an SQL and the sales department can start working on it.