92 is the atomic number - this refers to how many protons are in the nucleus, and hence is unique to each element. Therefore, any nucleus with 92 protons is Uranium.
However, an element can be in the form of various isotopes - an isotope is an element with a different number of neutrons to other isotopes. In this case, the isotope we are given is Uranium-238.
238 is the mass number, and refers to the total number of protons and neutrons within the nucleus (we take protons and neutrons to have the same relative mass of 1 each). Because we already know the number of protons, the number of neutrons is 238 - 92 = 146 neutrons.
I hope this helps
