In short
The chlor-alkali process is the electrolysis of concentrated sodium chloride solution (brine), which makes chlorine gas, hydrogen gas and sodium hydroxide.
The overall reaction is . Chlorine forms at the anode and hydrogen at the cathode, and the solution left behind is sodium hydroxide. The three products are all useful. Chlorine is used to treat water and to make PVC and bleach, and sodium hydroxide is used in making soap, paper and aluminium.
Passing a current through brine for one hour in a cell gives bubbles of greenish chlorine at the anode and of hydrogen at the cathode, and the electrolyte becomes alkaline.