what is property on Marx's view?

Private property is the right of an individual to exclude others use of an object, and predates the rupture of society into classes. In its undeveloped form private property is the simple relation of the individual to the natural world in which their individuality finds objective expression. Private property is essentially the denial of the private property of others and finds its ultimate expression only in the relation of wage-labour and capital.

“The antithesis between lack of property and property, so long as it is not comprehended as the antithesis of labour and capital, still remains an indifferent antithesis, not grasped in its active connection, in its internal relation, not yet grasped as a contradiction. It can find expression in this first form even without the advanced development of private property (as in ancient Rome, Turkey, etc.). It does not yet appear as having been established by private property itself. But labour, the subjective essence of private property as exclusion of property, and capital, objective labour as exclusion of labour, constitute private property as its developed state of contradiction - hence a dynamic relationship driving towards resolution.” [Private Property and Communism]

The abolition of private property constitutes the emancipation of humanity as the relation of person to person is immediate rather than mediated through things.

“Just as private property is only the perceptible expression of the fact that man becomes objective for himself and at the same time becomes to himself a strange and inhuman object; just as it expresses the fact that the manifestation of his life is the alienation of his life, that his realisation is his loss of reality, is an alien reality: so, the positive transcendence of private property - i.e., the perceptible appropriation for and by man of the human essence and of human life, of objective man, of human achievements should not be conceived merely in the sense of immediate, one-sided enjoyment, merely in the sense of possessing, of having. Man appropriates his comprehensive essence in a comprehensive manner, that is to say, as a whole man. Each of his human relations to the world - seeing, hearing, smelling, tasting, feeling, thinking, observing, experiencing, wanting, acting, loving - in short, all the organs of his individual being, like those organs which are directly social in their form, are in their objective orientation, or in their orientation to the object, the appropriation of the object, the appropriation of human reality. Their orientation to the object is the manifestation of the human reality, [For this reason it is just as highly varied as the determinations of human essence and activities] it is human activity and human suffering, for suffering, humanly considered, is a kind of self-enjoyment of man.

“Private property has made us so stupid and one-sided that an object is only ours when we have it - when it exists for us as capital, or when it is directly possessed, eaten, drunk, worn, inhabited, etc., - in short, when it is used by us. Although private property itself again conceives all these direct realisations of possession only as means of life, and the life which they serve as means is the life of private property - labour and conversion into capital.

“In the place of all physical and mental senses there has therefore come the sheer estrangement of all these senses, the sense of having. The human being had to be reduced to this absolute poverty in order that he might yield his inner wealth to the outer world..

“The abolition of private property is therefore the complete emancipation of all human senses and qualities, but it is this emancipation precisely because these senses and attributes have become, subjectively and objectively, human. The eye has become a human eye, just as its object has become a social, human object - an object made by man for man. The senses have therefore become directly in their practice theoreticians. They relate themselves to the thing for the sake of the thing, but the thing itself is an objective human relation to itself and to man, [in practice I can relate myself to a thing humanly only if the thing relates itself humanly to the human being] and vice versa. Need or enjoyment have consequently lost its egotistical nature, and nature has lost its mere utility by use becoming human use.” [Private Property and Communism]