There are two specifics here. On the first:-

"Der Junge hört zu" The boy listens to... You would need to add something here like 'der Musik'.

In the second one, Der Junge hört, it just means that 'The boy listens.'

"Der Junge hört zu" : 'The boy listens' (deliberate act)

"Der Junge hört" : 'The boy hears'

"Hoert" means "hears." "Hoert zu" means "listens." The difference is that you can hear something by accident, but if you're listening it means you're actively trying to hear.