Masking fluid (or frisket) is a liquid used to block out areas of a watercolor while you paint, thereby retaining the white of the paper or the previous color that was painted. It's a solution of latex in ammonia and is removed by gently rubbing it off either with your fingers or an eraser, once the painting is dry.
As it's tricky to get masking fluid out of a brush, it's advisable to applied it with an old brush or one kept solely for this purpose. Some artists recommend dipping a brush in washing-up liquid before you use masking fluid, as this makes it easier to wash out of a brush.
You can buy 'erasers' made from crepe rubber specifically for removing masking fluid; they look like a bit of a plastic from the insider of a shoe sole. (If you're searching for one on an online art supply store, try using the keywords "crepe rubber cement pickup".) Using one instead of your fingers to remove masking fluid has the advantage that you don't accidentally transfer grease or paint from your fingers onto your painting.
Masking fluid that's got a color is easier to use than one which is white or transparent as you can see where you've applied it. Permanent masking fluid is a special type of masking fluid, formulated to be left on the paper permanently.
Frisket film is a clear, low-tack masking film that can be used to mask out areas of a painting. You cut it to shape and stick it down on your painting. Ensure that the edges are stuck down so paint doesn't seep in underneath it.
