Masking fluid is a latex based substance that you paint onto your paper to prevent watercolour covering an area, thus leaving a white mark.
It smells like nappies and quickly dries in the bottle. These are the down sides.
The upside is that with a bit of planning you can guarantee that the areas you want as highlights will remain white.
When you've applied the masking fluid you have to let it dry naturally, if you use a hair dryer it will stick to the paper for ever! It usually takes about ten minutes.
The nice thing about masking fluid is that paint flows towards it and leaves a slightly darker soft line at the edge. It adds another dimension.
When everything is dry and finished you just rub it off with your fingers, or you can use an erasure but it's not so satisfying. If you leave the fluid on beyond a few days it gets difficult to take off.