The terms in-itself and for-itself were created by Sartre in Being and Nothingness.
They together form the ontology of Sartre’s theory of reality in Being and Nothingness.
The in-itself is the world of physical things (a paper cutter, an ashtray), fixed and static world in which thingshave an essence, that is to say a specific function.
The For-itself, in contrast, refers to the world of existence. The man is a being-louse itself, ie without fuel, it isa free existence thrown into the world. He has to build a gas.
Men, however, according to Sartre, constantly seeking to escape their situation through the pipes in bad faith (seeking to reify, to reify, to give a brief gasoline). Evidenced by the examples of the tune, love, the waiter.
In summary, the self is the mode of being of things, the for-itself mode of being.