Sartre is giving account of the person who attempts a removal of meaning to them, to arrive an account of existence itself.
Roquentin, who is searching of the meaning of existence, cannot find anything because any meaningful connection he experiences is of him. He is assaulted by Nausea in finding all the things he expects to be meaningful are not. In removing what he put with existence as it appears to him, he finds an empty world. Since he removed various meanings with existence
to his consciousness, he has nothing to see, hear, taste, touch, smell, etc. He has nothing to possess or seek. He has no-one to laugh with or love. He doesn't even have a sky too look at. In existence itself, there is no ocean and green, white spec or seagull. Positively sickening.
The Nausea is in coming a realisation the meaning of the object to oneself is located within one's own consciousness experience, rather than in the object--that one will never be able to find the meaning of the existence itself. The existence itself is empty of these human meanings and so cannot amount to a discovery of how the things "really work" or what they "essentially are."
Sartre is drawing a sort of similar point to Kant, only about meaning instead of epistemology. Since the meaning we experience is
our experience, the meaning of things cannot be given to us otherwise in any case. Just as what we know must be of experience for Kant, Sartre is pointing out the meaning we feel or encounter must be of our experience.
It seems as though Roquentin is having a kind of illumination, a positive revelation of the reality of the universe, whereas prior to his communion with the Nausea, he existed in a state of dull unreality.
Indeed so, existence itself has been illuminated to him. He now realises it is not merely equivalent to the various meanings which appear to him (e.g. green, ocean, white specs, sea gulls), but is instead something else which exceeds and is other to them. He learnt, for example, that there is more going on than just ocean [that] is green.
"Ocean is green" is merely one account of meaning which appears to him. Existence itself is much more. Not only a fact not mentioned in "Ocean is green," but a fact of consequence to existence--who knows what other meanings existence might be with? No longer can we say existence must necessarily be limited to the concepts of ocean in green. Someone else might come along and notice a purple ocean. Someone else a desert. And so on.