Saffron comes from a flower called the crocus sativus, and is the stigmas of the plant. This is why it’s incredibly expensive, as each plant only has a few stigma, and to cultivate 1kg of the spice requires over 100,000 flowers. Actually, the flowers only bloom for one week a year, and the saffron must be harvested by hand, in mid morning, so that the flowers are still closed to protect the stigma. The red threads give a yellow color to food, and impart a hard to describe floral, earthy, but absolutely irreplaceable flavor to dishes.
Since the spice is expensive, there’s a lot of fake saffron on the market as well. Some signs of fake saffron are a complete lack of flavor, or a bitter, metallic taste. If you were to soak saffron it should give off a yellow/orange color after 15 minutes. Now, fake saffron might also give off color, but the thread will have lost its color while real saffron will keep its color. That’s partially because fake saffron can be dyed.
Saffron is also slightly trumpet shaped, so one end will be larger than the other. If you were to rub the saffron between your fingers it should turn your fingers an orange/yellow color. If you were to put a saffron thread in your mouth and it tastes sweet, it might not be real saffron.