The nucleus (nuclei is plural) is the center of an atom.
It contains protons and neutrons.
Things to know:
Protons are positive and neutrons are neutral. Negative charge comes from electrons, which are not in the nucleus.
The mass of an atom is two times the mass of the protons or the mass of the protons and the neutrons added together, because they have approximately the same heaviness. The mass of an electron is so negligible they aren't factored in.