Occult (i.e. hidden, from the Latin verb occultare) knowledge is essentially philosophy through symbols that stretches back to the beginning of human history and that has been concealed from the great mass of humanity.
Occult rituals apply the philosophy of these esoteric symbols.
It was Confucius who said that it is signs and symbols, not laws or words, that govern the world.
Occultists learn about the meaning of these ancient symbols in their practice.
Dark occultists use their symbolic knowledge to manipulate others for their own selfish benefit.
De-occultists use their symbolic knowledge to make others aware of the hidden meaning of these symbols.
The word symbol comes from the Greek verb σύμβαλλειν which means to bring together.
Philosophy through visual symbolic representations stretches perhaps further back than philosophy through words, even though words are sort of symbolic too.
For example, both the $ sign as well as the € symbol have hidden (and dark) meanings.
So do numbers (see The Occult Significance of 9), corporate logos (such as the Apple logo which can be interpreted as being the forbidden fruit Eve bit in the Garden of Eden), street and building architecture (e.g in Washington D.C.), phallic monuments such as obelisks, tarot cards, police, military and graduate uniforms (see Graduates & Cops), street signs (such as the STOP sign in the USA), boxing rings (hypercube) and baseball pitches (freemasonic compass and square layout), governmental and monetary seals and regalia, mainstream popular music videos (see Satanic Insert in Beatles 'I Feel Fine' Video), movies, paintings and so on and so forth.
All that being said interpretations of these symbols are contested and contestable and do vary in quality.