Seasoned describes a person who has been around forever, doing what they do, and doing it well — throughout the seasons. They have lots of experience, and they can handle just about anything that comes their way.
To become seasoned takes a long time, because it means you are an old pro — someone who has dedicated years to a specific skill or activity. A seasoned hiker can tell you the fastest path up any mountain, and a seasoned bowler spends hours practicing in order to bowl strikes every time. Seasoned can also describe food that is full of herbs and spices, like the heavily seasoned French fries that seasoned bowlers eat at a bowling alley.