A metameric match occurs when two lights with different spectral properties (different wavelengths and or amplitudes) are perceptually identical to each other. Metameric matches are one line of psychological evidence that the human visual system is trichromatic.
In a classic metameric matching study people are shown one light. They then have to adjust the intensities of up to three other lights (typically a red, green and blue), the additive mixture of which should be a metameric match to the first color. Normal human observers require three lights to perform metameric matches. One cannot make a metameric match of all colors by adjusting the intensity of just one or two other lights. Adjusting the intensities of four lights is too many -- the metamer can always be done with just three. Three is the magic number because there are three distinct types of color sensitive photoreceptors -- the S, M and L cones.
A person who is missing one of their types of cones (a dichromat) can perform the metameric match by varying the intensities of only two lights. A person who is missing two types of cones (a monochromat) can perform the metameric match by varying the intensity of only one light. Theory predicts, and data agrees, that the number of types of cones that you have and the number of lights whose intensities you need to vary to make the metameric match should be the same.
Technically, to be a metameric match, you should adjust the intensity of the three lights so that the additive mixture (the wedge where the three circles overlap) perceptually match another light which has different spectral properties. Since all colors on your monitor are created by additively mixing the same three colors, the additive mixture and the to-be-matched color patch will have the same spectral properties (if you have correctly matched the colors.) To be technically correct, you should find an object in the real world and try to find the additive mixture that exactly matches it. That match is likely to be a metameric match. However, if you do so, there is no way of checking your results.
Thus, the activity allows you to match an additive mixture to another color patch. Adjust the brightness of the red, green and blue lights by moving the sliders left (dimmer) or right (brighter) until the wedge in the middle of the three lights matches the perceived color of the color patch. When you are satisfied that the additive mixture and the color patch are the same, you can objectively check your answer by clicking on the "Show RGB" button which will display the proportion of red, green and blue in the to-be- matched color patch. You can compare those values with the proportion of red, green and blue that you adjusted the additive mixture to.
Note that adjusting the additive mixture to the values given when you click on the "Show RGB" button may or may not make the additive mixture perceptually identical to the to-be-matched color patch. Can you explain why? If you don't believe that they are the same physical color, you can click on the "Show color under mouse" button which will show the proportion of red, green and blue under the mouse when the mouse is hovering above the additive circles or the to-be-matched color patch. If you still do not believe it, get a piece of heavy paper and punch two holes in it -- one over the additive mixture and the other over the to-be-matched color patch. Cover up the display with the piece of paper and look at the two colors through the holes. When the surrounding colors in the additive mixture are removed, the colors appear identical. That might give you a clue as to why the phenomenon arises in the first place.