The second nave window

In this window, the chalice and the host (consecrated bread) represent the Eucharist (Holy Communion).

The Latin word calix means cup and hostia is Latin for victim –
“For I received from the Lord what I also delivered to you, that the Lord Jesus on the night when he was betrayed took bread, and when he had given thanks, he broke it, and said, ‘This is my body which is for you. Do this in remembrance of me.’ In the same way also he took the cup, after supper, saying, ‘This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me.'” [1 Corinthians 11:23-25].

The letters IHS on the host are the first three letters of the Greek name for Jesus (see HERE for an explanation).