The objective in hockey is to outscore the opposition. A derivative of that objective would be to create lots of scoring opportunities for your team while preventing the other team from creating many of their own. One more derivation might be: get control of the puck and push play towards the opposing end of the ice.
If you have the puck, then your efforts are going towards advancing the puck forward to the offensive zone. If the other team has the puck, then you want to stop them from advancing it and ultimately force them to give it up.