The law of constant proportions states that chemical compounds are made up of elements that are present in a fixed ratio by mass.
This implies that any pure sample of a compound, no matter the source, will always consist of the same elements that are present in the same ratio by mass.
For example, pure water will always contain hydrogen and oxygen in a fixed mass ratio (a gram of water consists of approximately 0.11 grams of hydrogen and 0.88 grams of oxygen, the ratio is 1:8).