Hiking refers to walking in a natural environment often in mountainous areas and along trails or paths purposely made for walkers. You cannot go hiking in the city! Afternoon refers to anytime after midday. But since a hike is usually an extended walk it would normally be for more than just an hour. In the United Kingdom we also call it 'rambling' and long distance 'hiking' with overnight stops is also called 'trekking'.

It can also mean an afternoon walk after school on weekdays or after lunch on weekends. People may just go for a walk around their suburb or a local park. Others go to nearby mountains or lakes. Here's an actual example of an afternoon hike http://www.youtube.com/watch?v=cNsKuriyfJQ