Something contextual relies on its context or setting to make sense. If you touch someone and shout "You're it!" in a game of tag, people get it, but if you're in the grocery store tapping strangers on the shoulder and yelling at them, it's less contextual.
You can use the adjective contextual to describe what something means as it relates to a place, or meaning in a written text. If someone asks you what contextual reason you have for choosing an answer after reading a chapter, for example, you'll have an opinion in the context of what you read — it is contextual because it came out of the text. Activities are contextual too: riding a skateboard in a dance studio is not contextual, but doing ballet there would be.