Atoms are the basic building blocks of all matter (solids, liquids, gases and plasma) in the universe. For example, you and I are made entirely of atoms. The device you're using to view this answer is made of atoms.
Within atoms are subatomic particles. These include protons, neutrons and electrons. Protons and neutrons are located within the nucleus of an atom while electrons orbit around the nucleus, arranged in shells (the "outside part" of an atom). Changing the number of subatomic particles there are in an atom can change its properties (ex. what element it represents, how reactive the substance is, etc.)
Please check the image attached for a visual representation.
I hope this helps!