Masking Fluid with Watercolor
I seldom use it, but the brand I prefer is Winsor Newton Liquid Masking Fluid (with the yellow color so you can see where you have painted it on the page). It should be applied to totally dry paper, and allowed to dry totally before applying paint over the top.
It is used for keeping the white of the paper in difficult areas, so that you can paint freely without destroying the white. However, when removed (either by rubbing it, or using a rubber cement eraser) the edges are never quite what I thought they would be. Then I have to go back in with a scrubber to adjust the edges. I might as well have painted around the whites to begin with!
It also can be splattered to maintain small spots of white (like in a snow scene) or other such subjects.
For sharp, clean edges, I apply it with a ruling pen. Ruling pens were very popular before the computer took over drawing straight lines, but they can still be found in Office Supply stores and some Art stores too. You don't need the most expensive kind, so don't break your bank account for the use of masking fluid. Remember, tho, to clean them thoroughly and dry them when finished.
I do not recommend Permanent Masking Fluid, as you cannot remove it and may need to! I have never found a good use for it.
Learning the Basics of Watercolor
Painting Flowers in Watercolor
Painting on Watercolor Canvas