Use directly as masking fluid or use this bottle to refill your Masking Fluid Marker!
Liquid frisket, masking fluid, liquid mask (okay I made that last one up) are all the same thing. A way to apply a fluid that, once dry, will act as a removable resist to water-based media.
From the manufacturer:
Masking without leaving any residues. Application of liquid masking fluid has never been easier. Draw precise lines with the ready-to-use marker, without using a brush. Like this, white outlines and gaps become visible on nearly all surfaces. Painting over the dry masking liquid is therefore possible with nearly all acrylic, water or alcohol-based inks. Masking liquid can be removed without leaving residues for two days after drying.