There are three things about the occult:
First, it’s about symbolism—but in an unusual way. The symbol does not just indicate the thing, it affects the thing. A regular symbol is like: the eagle represents America. If hurting the eagle hurt America, then you’d be moving toward the occult.
Second, while the system of symbols is (this is the literally meaning of the name) hidden—it is often hidden in plain sight. This is perhaps most purely expressed in the sentence from Twin Peaks “The owls are not what they seem”. The occult indicates a series of hidden connections between seemingly mundane things—it doesn’t just involve esoteric words and phrases, it uses ordinary words and phrases as if they had a significance we don’t give them.
It is, in this way, simply a parallel alternative to scientific explanations. Science claims that a pencil lead and a diamond have a secret connection (they are both carbon), the occult merely claims a different correspondence: this triangular window over here has a hidden connection to that man’s eye over there. Once the system of connections is understood, the whole world looks different: the investigator is living in a world full of levers to push and knobs to twist.
Third, the system of symbols is old. It has been hidden (occulted) for a long time. In order to charge, say, an inverted triangle with occult flavor you don’t have to immediately give it a sinister meaning—just give it a meaning that goes back to ancient Babylon.