This is the 938th most frequent Spanish word.
contrario
Opposite.
Here, 'contrario' is used as an adjective meaning 'opposite', describing the spatial relationship of the sides of the field.
El equipo visitante jugó en el lado contrario del campo.
The visiting team played on the opposite side of the field.
In this sentence, 'contraria' is an adjective and means 'contrary', describing the opposition or difference between opinions.
Aunque lo intenté, mi opinión era contraria a la de ellos.
Although I tried, my opinion was contrary to theirs.
'Contrario' here is used as a noun, meaning 'the opposite', referring to an action that goes against expectation.