In order for your brakes to function, you need to apply brake fluid on a regular basis. Brake fluid is a sort of lubricant that lessens friction from different parts of your brake system rubbing together when the brakes are activated.
Substances that are used in making brake fluid are often hygroscopic, meaning that they absorb water over time. This can cause your brakes to rust if your brake fluid is not changed regularly. Changing your brake fluid requires a process called "bleeding your brakes," meaning that you have to drain the old fluid before applying the new.
Common types of brake fluid include DOT 3, 4, 5, and 5.1, and each of these types differs in their chemical make-up, so it is generally not a good idea to mix them together unless it is absolutely necessary, and your car will almost certainly require one specific type of brake fluid.