WHAT IS IT?  A time averaged exposure to a concentration of airborne contaminant.

Example (very simple):  exposure to 2000 pm acetone for 3 h, followed by no exposure for 5 h....

(2000 ppm)(3h) +(0 ppm)(5h)     =  750 ppm 8-h TWA
                   8 h

(The PEL of acetone is 1000 ppm, 8-h TWA.)