This law states that "matter cannot be created or destroyed". Atoms cannot appear or disappear into thin air. In chemical reactions they can only be rearranged.
The number of atoms before and after a reaction must be the same.
The mass before and after will weigh the same.
Chemicals have the same proportions of elements no matter how they are formed.
For example, water,H2O, is composed of 11.19 %H and 88.81% O by mass. All water in the universe will have this compostion.
Pure elements are said to be in their "elemental form". The chemical formula of most pure elements is simply their elemental symbol. However, some gaseous elements form molecules made of two of the same atoms.