Explanation: “To make ends meet” is an expression used when someone is working hard to earn enough money to pay the bills. The word “poor” in “poor dog” can be an expression of sympathy such as “you poor thing” or it can mean poor as in a person with little or no money.
This dog is working to make ends meet by having its mouth grab its tail. If and when the dog bites its tail, its head will literally meet the tail; the two ends of the dog will meet. This joke is funny because it plays with the word “poor” and the expression “make ends meet.”