We don't use the word "reck" anymore, but to reck things meant to be thinking about them: to be careful about them, to be aware of them. We DO use the word "reckless," which means "NOT careful about things." If you're being reckless, you're acting in a fast, dangerous way, because you're NOT thinking carefully about what you should do or how you should stay safe.
This bus driver is reckless. He's driving recklessly: way too fast, and the bus is in danger of crashing. His reckless driving might injure or kill all of them.
This diver is being reckless: chasing after a shark, trying to grab it. He could injure the shark. And, the shark could turn right around and bite him. Chasing after a wild animal is a reckless thing to do.