This is not very specific, so I will give a general explanation of the word "light" to make sure I've answered your question properly.

Light, as in visible light, is a certain small band in the electromagnetic radiation spectrum. Electromagnetic radiation is a pair of perpendicular magnetic and electric fields travelling in tandem through space. Due to quantum weirdness, it is equally well described as a particle (called a photon) travelling through space. This has little to do with cats, which seems to be what comes to mind when most people here the words "quantum weirdness". Resist such foolish thoughts.

Light, as in the visual sensation of light, is caused by chemical reactions in your retinas caused by electromagnetic radiation in the visible spectrum hitting them. Certain frequencies set off reactions in different cells by different amounts ("rod" cells respond purely to brightness, while "cone" cells respond largely to the chroma). For something that produces images as pretty as it does, this is a rather unromantic description, and if you feel that it has removed joy from your life, you should have skipped it. Don't say I didn't warn you because we all know I didn't already.

Light can also be considered in a metaphorical context, where it is often associated with knowledge, morality, reason, and other positive concepts. Basically it's associated with "the good guys", which is weird because everyone knows that black is the coolest colour.

Light can also be used as an adjective to describe a colour that is paler than usual, or to describe something with comparatively little mass. I say comparatively, because it's quite subjective and what is laughably light to me might very well be quite heavy to you.

I think that about covers it.