The curve you see is a plot of the photopic luminosity function.
If you look at two same-size spots of light with the same power in each, but differing by wavelength, their brightnesses will differ. E.g. if you shine a 405 nm laser pointer at a white wall, and a 532 nm pointer of the same power (e.g. 1 mW) near it, you'll notice that the spot of the former seems duller than that of the latter, despite the power being the same. Moreover, if you try the same experiment with e.g. 1 mW of 808 nm laser pointer, you'll most likely not even notice the spot, except when looking at the reflection at a special angle in a dark room.
The luminosity function is a way to quantify these "amounts of visibility" of different wavelengths. It appears that the human eye in photopic (daylight) vision mode is most sensitive to 555 nm light, and the sensitivity decreases away from this wavelength. This is reflected in the maximum of the photopic luminosity function.