In string theory, generally called superstring theory, the fundamental consitutent is a two dimensional string...kind of a stretched rubber band like strand which is actually a vibrating filament of energy. Properties we observe at macroscopic levels reflects the nature of the string vibrations. The detailed pattern of vibration executed by a string produces a specific mass, a specific electric charge, abnd a specific spin...and so on.
A string with highly active vibrations has more energy and from E = mc2 type cosniderations, is observed to have more mass. All particle properties are determined by the vibrational patterns of strings.