Question

What is the $2$ in the ${H_2}O$.

Answer 1

${H_2}O$ is dihydrogen monoxide or simply water. The 2 is the subscript of H, which represents the presence of 2 Hydrogen atoms. A subscript is a character, usually a letter or number, that's printed slightly below and to the side of another character. Subscripts are commonly used in chemical formulas. It's closely related to superscript, when a letter or number is printed above another character.

Complete answer:
Chemical formulas use letters and numbers to represent chemical species. The numbers appearing as subscripts in the chemical formula indicate the number of atoms of the element immediately before the subscript. If no subscript appears, one atom of that element is present.
We use subscripts in chemical formulae to indicate the number of atoms of an element present in a molecule or formula unit. It indicates that each water molecule contains two hydrogen atoms and one oxygen. In terms of atoms, water always has a $2:1$ ratio of hydrogen to oxygen.
The chemical formula of water is ${H_2}O$. It is also called dihydrogen monoxide. The $2$ is the subscript for $H$ symbolizes the presence of two Hydrogen atoms. Thus, we can say that there is hydrogen bonding in water. Therefore, water contains two hydrogen atoms and one oxygen.

Note:
Remember subscripts and coefficients are two different things. The key difference between coefficient and subscript is that coefficient gives the number of moles of a substance, whereas subscript gives the number of atoms present in a molecule. Both these terms refer to numbers, but they give different details about a particular chemical reaction.