What does light-struck mean?
This is when the beer has been exposed to ultraviolet light for a period of time. Hop-derived molecules, called isohumulones, are basically ripped apart. Some of these parts bind with sulfur atoms to create that “skunk” character, which is similar in character to a skunk's natural defense and is such a potent compound that parts-per-trillion can be detected and even ruin a beer. Although brown bottles aid in protecting beer from being light-struck, it hardly makes the beer invincible. Green or clear bottles provide little to no protection. And it's been said that bottled beer can become light-struck in less than one minute in bright sun, after a few hours in diffuse daylight, and in a few days under normal fluorescent lighting.
This light-struck condition is often to blame for a skunked beer. Many popular imports are distributed in green bottles. Pair the two, mix with misinformation and an inexperienced palate, and the common belief becomes that if beer is in a green bottle, it has to be skunked.