The Wavelength of light
1) It means the the displacement from crest to crest (in phase points) in the electric or magnetic wave is 750nm, in the measured medium (a vacuum in this case).
2) Certain wavelengths can penetrate further than others (x rays vs visible light for example). In a vacuum the weve would go on indefinitely as no energy is lost in the moment (using the partical modal of light). The oscillations effect the frequency of the wave therefor effecting the wavelength (c = f*lamda, where lamda is the wavelength and c is the speed of light). the actual distance will depend on how energy is lost due to friction and induction with the surrounding medium.