When hops are boiled down to make beer, they release chemical compounds called Iso-Alpha Acids. They're bitter on their own, but when exposed to sunlight they break down and interact with other molecules in the beer to produce a molecule that's almost identical to the one in a skunk's smelly spray.
We're exaggerating, but Corona and other beers bottled in clear or green glass have developed a reputation as smelling “skunky” or spoiled. The leftover chemicals hook up with sulfur-packed proteins, and produce a chemical compound that's structurally (and smellably) similar to a skunk's defense spray.
A beer becomes “skunked” or “lightstruck” when it has been exposed to light for a period of time. While it is not dangerous to drink a skunked beer, it's pretty disgusting to smell and taste. You were meant to enjoy beer, so don't drink skunk your beer!