This is sort of the heart of it. It doesn't particularly mean draw quickly per say but produce quickly.
You work slower but reduce the scope of a MVP (minimal viable product), so anything you have to change can be addressed quickly without compromising the overall quality.
So if you specialize in coloring faces and you want to draw a happy person, you say your minimal viable product is "drawing a smiling colorful face". So you define the face in the sketch and go straight to coloring it spending as much time mastering the facial expression and impression. So you are thus creating something high quality quicker by effect. It just doesn't have a full landscape or maybe full-body.
In this way your passes aren't defined by
1. sketch everything 2. rough everything 3. color. everything
It's more
1. sketch & color face 2. sketch & color shirt 3. sketch and color horizon at the head area
In theory you could ship this at any pass and it has your signature style and a high quality production.
It just may not have everything it could be.
In the waterfall method when you come to a deadline you usually have to choose between
a. doing a rushed job but getting everything done,
b. or having something that works well in one area but is compromised by unreliable features or missing important requirements. And if you have no deadline you get "scope-creep" which means the more time spent on the project, the more requirements will be discovered or revealed.
Agile just means get ready for the future without planning for it. The methodology is getting to your expertise as quickly as possible and applying to to the heart of the project. Take as much time you need so you're work can be expanded or perfected as you see fit.