Best Reward [Joke]
A naval officer fell overboard. He was rescued by a deck hand. The officer asked how he could reward him.
"The best way, sir," said the deck hand, "is to say nothing about it. If the other fellows knew I'd pulled you out, they'd chuck me in."
If you fall overboard, you fall over the side of a boat into the water.
A reward is something that you are given, for example because you have behaved well, worked hard, or provided a service to the community.
Your fellows are the people who you work with, do things with, or who are like you in some way. (FORMAL)
When you chuck something somewhere, you throw it there in a casual or careless way. (INFORMAL)