What does it mean if you put the middle finger up in Islam?

It is not the middle finger.

The hand fingers are called:

1. thumb
2. the index finger (next to the thumb)
3. the middle finger (in the middle)
4. the ring finger (though this is customarily used for the left hand only)
5. the "little finger" or the "pinkie" (the smallest and last finger)

Usually, Muslims put the the index finger (next to the thumb) up when viewing a funeral and during praying as sign that Allah (or God in English and same God worshiped in Christianity and Judaism) is one and only one God with no partner.