To believe something is true without knowing it directly or without it being proven or demonstrated.
That's the dictionary definition, but the course goes further. Let me explain. According to the course, belief implies acceptance of its existence. That's why you can believe what no one else thinks is true. For you, it's true because you created it. (T1) And it adds: to believe is to accept and also to side with what you accept. Believing is not being gullible, but accepting and appreciating. (T9)
You have to question all your beliefs, not only because you created them but because they may come from someone else and you accept them. The problem is that for that person it may or may not be true, but they want it to be true for you. Does this sound familiar to you, like journalism? Religions? Or whatever. Practice doubting every belief you've ever held. The question is: Is this true? Who says so? When you form a belief based on something that isn't true, your way of thinking and acting will be incorrect. I don't need to give you examples. Very often, the world tries to make you believe its story. This is because they have interests, and someone has to satisfy them.
Question every belief, and even those you question with a little critical thinking, don't forget that when there is doubt, there is no doubt.
José Omar Gastaldi