"He goes, I go."
In French, you have to give a hint on your destination with verb "aller". "Y" is a convenient pronoun meaning "there". There is no context here but if the beginning of the discussion was about going to a specific place, "y" would replace that place without repeating it:
- tu doist aller à la poste avant qu'elle ferme (you must go to the post office before it is closed)
- oui, j'y vais = oui, je vais à la poste (yes I go -to the post office)