5.29 describe the manufacture of sodium hydroxide and chlorine by the electrolysis of concentrated sodium chloride solution (brine) in a diaphragm cell

Brine is NaCl solution.
In electrolysis, chlorine is created at the anode, hydrogen is created at the cathode, sodium hydroxide is left in the solution.

2NaCl + 2H2O > 2NaOH + H2 + Cl2

This happens in a diaphragm cell.