An atom is the basic unit of an element. When you see the chemical formula for water, H2O, it's telling you that each molecule of water is made up of two atoms of hydrogen and one atom of oxygen.
In science class, you've probably come across atoms, the bits that make up molecules. Less scientifically, the word atom can also mean a very small piece of anything at all. The Greek root of atom is atomos, which means "indivisible," since the scientists who first gave the atom its name imagined it couldn't be split or divided into smaller pieces. Even though we know there are smaller things that make up an atom (protons, neutrons and electrons), it's still a good way to think about the word atom.