All substances are made from atoms. And any given element, such as copper, is made of atoms of just one particular sort.
The atoms of any element are different from the atoms of any other element. So iron contains a different sort of atoms from those of sulfur, and the atoms in carbon are different from those of oxygen.
The atoms of each element are represented by chemical symbols. These usually consist of one or two different letters (sometimes three letters are used for newly-discovered elements).
The first letter in a chemical symbol is always an UPPERCASE letter, and the other letters are always lowercase. So, the symbol for magnesium is Mg and not mg, MG or mG.
Every element has its own chemical symbol. For example,
copper is Cu, iron is Fe, sulfur is S, sodium is Na and oxygen is O.