This when a subject is defined by painting around the the subject to create it’s shape as opposed to painting the subject itself.
As in the beautiful painting on the right by Krista Hasson, the daisies are just mainly the white of the paper and the outline is produced by painting the background. This is commonly used in watercolour painting as a light colour cannot cover a darker colour.
Going around the subject can be extremely difficult if you are trying to produce a smooth background. One of the techniques that can help with this is applying Masking Fluid otherwise known as Liquid Frisket. This is applied over the subject and allowed to dry, a wash is then applied over the top, when the wash is dry the Mask can be removed.
There are many pros, cons tips and advice to using masking fluid which will be covered on another page at some point.