You're likely thinking about chemical formulas, which use symbols and numbers to represent the composition of elements and compounds.

Here's how they work:

* Element Symbols: Each element is represented by a one or two-letter symbol (e.g., H for hydrogen, O for oxygen, Na for sodium).

* Subscripts: Numbers written as subscripts after an element symbol indicate the number of atoms of that element in the compound. If there's no subscript, it's assumed to be 1.

Examples:

* H₂O: This represents water, which has 2 hydrogen atoms (H₂) and 1 oxygen atom (O).

* NaCl: This represents table salt, which has 1 sodium atom (Na) and 1 chlorine atom (Cl).

* CO₂: This represents carbon dioxide, which has 1 carbon atom (C) and 2 oxygen atoms (O₂).

Let me know if you'd like more examples or explanations of specific chemical formulas!