Now, if I go farther, and for weighty reasons rank as mere appearances the remaining qualities of bodies also, which are called primary, such as extension, place, and in general space, with all that which belongs to it (impenetrability or materiality, space, etc. )—no one in the least can adduce the reason of its being inadmissible." This follows directly from Kant's transcendental idealism, according to which space and time are mere forms of intuition, which means that any quality that can be attributed to the spatiotemporal objects of experience must be a quality of how things appear to us rather than of how things are in themselves.
The passage explains that according to Kant, space and time are not real entities but rather how we perceive them through our senses. The qualities of bodies like extension, place, and materiality are only apparent because they are perceived by us.