The present tense is used to say what you do routinely, or what you do as a hobby:

On Tuesdays I go to Alaska and sing dirges to the killer whales.

I eat chocolate chip cookies every day.

I work out three times a week.

The present perfect tense is used to describe what is happening at this very moment, or this general period of time:

That boy is throwing cookies at the ducks.

I am studying English at Ohio State. (compare that to "I have English class every Tuesday at 3 p.m.)

I am sitting at my computer typing this sentence.

So, in your example, Matthew is doing something right now.

Hi Mathew!  What are you looking at?

What do you look at?

This is in the present tense, so this would show habitual action, or routine:

Mathew:  Hi, Emma!  Guess what!  Everyday I go and sit by the pond and just look.  I look and look, and then I look some more.

Emma: Really?  That is fascinating!  What do you look at?

Mathew: I look at the ducks.  I look at the little waves in the water.  I look at the water lilies.