This test is called the duochrome test, it is used to estimate whether you are long or short sighted (or whether the lenses you are wearing are too weak or too strong).

This 'red and green' test works on the principle that the wavelengths of red and green light are focused slightly differently by the eye, red being closer to the retina with short sight (i.e. clearer) and green focused closer to the retina in long sight.