In the electrolysis of water, $\ce{H2O}$ molecules are being reduced at the cathode (forming $\ce{H2}$ and $\ce{OH-}$) and oxidized at the anode (forming $\ce{O2}$ and $\ce{H_3O+}$).
It is said that:
The electrolysis of water usually involves dilute, or moderately concentrated, salt solutions in order to reduce the power loss driving the current through the solution, but the presence of salt is not a requirement for electrolysis.
What is the reason for "power loss"? How does the presence of foreign ions change that?
The electrolysis seems possible to happen in pure water alone (as the quote also says).