What is forward masking I am trying to understand what is visual masking (or more specificaly forward masking). I have read the explanation here, but still..

I understand that a background can interfere with a frontal visual stimulus. Anyone knows that. If I use whithe letters on white page I will see nothing. But, I assume the definition has more then that.. for example, in here, in figure 1b, are they describing an experiment? If yes' then what is the experiment? I mean, what is the stimulus, what is the masking, what is the experiment question, and what is the result (I assume that there are known results to what the figure is describing...)

Thank you!!
Visual masking has quite a few variations, and the source you've linked is by far the worst explanation I've seen.

In forward masking an early stimuli takes hold over consciousness (resulting in top-down activation) that prevents a later stimuli from overshooting the selective attention threshold, thus the latter remains subliminal (limen is threshold in Latin, so sub-liminal is literally below threshold).

For example, in the following frame sequence you'd be asked to find the letters in a sequence of numbers. Given there's less than 100ms between the two targets, your conscious attention to the first target will almost always prevent the second target from being perceived.



For a nice demo of this have a look at this attentional blink test video.

Another variation is to expose a frame for less than 30ms between various pre or post masks.

The principle, however, is always the same - paying attention to one stimuli 'saturates' our consciousness and prevents another from entering it.