But what is
It's a grammatical ending for adjectives (genitive singular for masculine and neuter nouns), the full form is -(i)ego
Think of it as meaning 'of'
II-go (drugiego) of the second
III-go (trzeciego) of the third
I understand that this means
the first course was
Actually it doesn't..... it means "of the first course were" which doesn't make much sense without more context...