Atoms consist of electrons and protons (and neutrons, but that is a separate issue). An atom is considered electronically neutral because it has equal quantities of electrons and protons. The protons are in the nucleus. The electrons are circulating around the nucleus. The electrons may separate. Heat from a dryer is one such cause of separation. These separated electrons are considered static electricity.
After being tumbled in the heat of a dryer, clothing is no longer likely to be electrically neutral. Rather, some pieces are positive, whereas others are negative. The need to achieve neutrality causes the clothing pieces to cling together----opposites attract.