Water is a molecule that is made up of two Hydrogen atoms (H) and one Oxygen atom (O) bonded together. The molecular formula H2O is representative of this bonded molecular structure, which is often used as an abbreviation or synonym for water.
On the Periodic Table of Elements, each atom is listed using its symbol, not its name. In this case, H is the symbol for the Hydrogen atom, and O is the symbol for the Oxygen atom. The number 2, typically written as a subscript, indicates that there are two Hydrogen atoms in this particular molecular structure. Put together, they comprise the alphabetised symbols for the molecular structure of water.