The word joint has several meanings:

1. A place where two things connect or meet. Your elbow is a joint where your upper and lower arm bones meet, allowing your arm to bend. A door joint is where the door attaches to its frame with hinges. Builders and carpenters pay careful attention to joints when constructing furniture or buildings, because a weak joint means the whole structure might fail. The phrase out of joint means something is dislocated or not working properly, like when a shoulder pops out of its socket.
2. Something shared between two or more people or groups. A joint effort means people are working together toward the same goal. When two authors write a book together, they might create a joint work. If your family makes a decision together, you might call it a joint decision. The word emphasizes cooperation and shared responsibility.
3. In casual speech, people sometimes use joint to mean a place or establishment, like calling a restaurant “a burger joint.” This usage is informal and friendly, suggesting a casual, unpretentious spot.