An olive starts out green. As it ripens, it turns pink, then purple, then almost black. The ones you eat as black olives are just ripe; the green ones in jars were picked early. They are too bitter to eat straight off the tree - they have to be soaked in salty water for weeks first.
Most olives are pressed into oil instead. To make olive oil, the olives are crushed (the old way was a giant round stone rolled over them), then the juice is left to settle. The clear golden oil floats to the top. Greece is one of the biggest producers of olive oil in the world.
Olive trees can live an incredibly long time. The oldest in Greece is on the island of Crete, in a village called Ano Vouves. Scientists think it is around 3,000 years old. It still grows olives every year. That tree was alive at the same time as Odysseus might have been - though Odysseus is from a story and the tree is real.
In ancient Greece, olive oil was used for cooking, for lamps to light the night, for soap, and to rub on the body before sports. The winners of the very first Olympic Games were given a crown of olive leaves instead of a gold medal. Even today, you might see Greek people wearing a tiny olive leaf at a special occasion.