Uranium, atomic number 92, has several isotopes, and 238U is just one of them. Remember that a nucleon is one of the particles that make up the nucleus of an atom, and that means a proton or a neutron. In the case of this isotope of uranium, it has the 92 protons we'd expect for uranium, and it has 146 neutrons in its nucleus along with those protons. That's 238 necleons in the nucleus if 238U. Wikipedia has more information on uranium and on the nucleon, and links are provided.
most of an atoms volume is nothing, think of the atom as a football stadium, a football in the centre is the neucleus, electrons would be the size of peas in the very back row, the rest is empty space. on top of that the neclues is made of protons and neutrons, which spaces between, these necleons are made of quarks which also have empty space between them, the quarks are made of neons which again have spaces between. Many physicists believes this continues in this fashion.