What does it mean when a small number is placed at the bottom right of another number?

A subscript is a small number (or letter) placed below and to the right of another number (or letter). Subscripts can mean many different things in different fields of endeavor.

In Chemistry the components of a molecule are designated by a letter indicating an element and a subscript indicating the number of those atoms in the molecule (Water = H2O = two hydrogen atoms and one oxygen atom in each water molecule)

Subscripts are often used to refer to members in a mathematical sequence or set. For example, in the sequence O = (44, -23, 560), O3 refers to the third member of sequence O, which is 560.