The elements are made up of atoms.
Atoms make elements and the atoms of different elements can combine to make compounds. More on that in a bit.
All the atoms of an element are chemically the same as each other so every atom of iron will react like all other atoms of iron. A chunk of pure gold (an element) has atoms that are all the same from a chemical standpoint.
Atoms are made of protons, neutrons and electrons. It is the number of protons that determines what element it is. If the atom has one proton, it is an atom of hydrogen. If the element has 8 protons, it is oxygen. If the atom has 26 protons, it is an atom of iron.
When two atoms of hydrogen react with an atom of oxygen and chemically bond, we get water,