Complex sentence: In order to use geometry to study the movement a mechanism, its links are modeled as rigid, which means the distances between points in the link do not change, that is the link does not flex, as the mechanism moves.
Split into simpler sentences: In order to use geometry to study the movement a mechanism, its links are modeled as rigid bodies. This means distances between points in a link do not change, that is the link does not flex, as the mechanism moves.