Complex sentence: The General Conference on Weights and Measures (or CGPM) is the convention's principal decision-making body, consisting of delegates from member states and non-voting observers from associate states.
Split into simpler sentences: The General Conference on Weights and Measures (or CGPM) is the principal decision-making body put on place by the convention. It is made up of delegates from member states and (non-voting) observers from associate states.