Answer:
It means that it always contains certain elements and it always contains elements in definite proportions.
Step-by-step explanation:
A mineral has five characteristics: definite chemical composition, naturally occurring, inorganic, solid and ordered internal structure.
Definite chemical composition means that every time a mineral is naturally created, its chemical formula is always the same, there are no modifications on the number of atoms of the elements that are contained in it, and those elements do not change.