Before we discuss Subjects, it would be remiss to not highlight, they have use cases but beginners often use them for the wrong ones, and end up in convoluted situations. As you will learn, they are both an Observer and an Observable, acting as a proxy mulitcasting device (kind of like an event bus). They do have their place in reactive programming, but you should strive to exhaust your other options before utilizing them. Erik Meijer, the creator of ReactiveX, described them as the "mutable variables of reactive programming". Just like mutable variables are necessary at times even though you should strive for immutability, Subjects are sometimes a necessary tool to reconcile imperative paradigms with reactive ones.
But before we discuss when to and when not to use them, let's take a look at what they exactly do.
