If you give a couple of different examples, I'm sure you'll get some analysis for them, which will hopefully help.

Do you mean pointing like Jesus is in this painting? This is a sign of benediction, indicating a blessing.

If you mean a pointing hand like this one seen in an Italian manuscript, the finger is indicating a particularly important line of text.

I'm sorry these aren't really in-depth answers, but I'm afraid I'm not sure what you mean by pointing. Could you link some examples?

What kind of art? Religious art? Illuminated manuscripts? Portraiture?

What time period? What region or regions? The Middle Ages covers a span of roughly a thousand years and thus medieval art covers a wide variety of art styles. Many art styles are regional, and this becomes especially important during the Middle Ages, when different parts of Europe were at different points in civilizational and therefore cultural development.