A vegetable oil popular in the Mediterranean. It's made from pressing olives and filtering the result.
There's multiple quality gradations of olive oil. The main ones are the extra virgen olive oil which is the first pressing of the olives, it's also pressed without aditional heat so all the flavors are preserved.
The other common quality gradation is just regular "olive oil" without qualifiers, which is the result of pressing the pulp from the first pressing again with a heated press. It's a lot cheaper if you need larger quantities for a dish but it has a lot less of the olive oil flavor you might need.
Don't heat olive oil above 190°C or 374°F, this is the point where it will start smoking and it will become bitter and will create some harmful compounds that might be unhealthy.