A fluorescent spray paint used by construction workers to draw the eye to an object that may otherwise blend into the background. It's often used to mark new crossbeams at head height.
Safety orange is named for its colour; other items the same colour include Sam Browne belts worn by cyclists and the high-visibility bibs worn by roadside workers. Dyno-Rod vans are a similar colour, though more eye-jarring.
Safety orange is great stuff for spraying Stanley knife handles, so that you can find it when you've put it down. It also makes an attractive respray for your laptop. If you spray your bike with it, you will never get SMIDSYed again.