Complex sentence: The hymns were essentially texts that paraphrased the particular action of the Mass happeneing at that time e.g. during the Kyrie eleison the congregation would sing a hymn asking God for mercy.
Split into simpler sentences: The hymns were essentially texts that paraphrased the particular action of the Mass happening at that time. During the Kyrie eleison, for instance, the congregation would sing a hymn asking God for mercy.