His powers are, more or less, weather manipulation, very similar to those shown in the pilot by his brother, Clyde.
The site summarizes his story to be one of revenge for the death of his brother, Clyde Mardon, who manifested the same powers but was killed in the pilot episode. If you remember, it was Joe that killed Clyde, so Mark Mardon is coming for him.
According to the site, he’ll be making his appearances in episodes 15 and 16 later in the season.
The Flash returns January 20th on the CW.