The zeroth law of thermodynamics says that if you have three things, and the first thing is the same temperature as the second thing, and the second thing is the same temperature as the third thing, then the first thing is also the same temperature as the third thing.
For example, let’s say you have a cup of hot cocoa, a bowl of soup, and a glass of ice water. If the hot cocoa and the soup are the same temperature, and the soup and the ice water are the same temperature, then the hot cocoa and the ice water are also the same temperature.
This might seem like a common-sense idea, but it’s actually an important principle in thermodynamics because it helps us define temperature and understand how heat moves between different objects.