I don't understand the withdrawal rate  why lower withdrawal rate = longer time to retire?

Adminkablamo (Admin, Networthify) commented
The withdrawal rate is the rate at which you withdraw money from your nest egg (portfolio) after you retire. For example, your portolio may earn an avg annual return of 7%. But you decide to only with draw 4% each year and reinvest the remaining 3%.
If you save $1 million and withdraw 4% each year you will get $40,000 to spend each year.
If you save $1 million and withdraw 3% each year you will get $30,000 to spend each year.
The calculator assumes your current expenses are the same as your expenses after retirement. I generally want to live have the same standard of living after retirement as I do now.
So if you spend $40,000 a year want a withdrawal rate of 4% you will need to save $1 million to retire. If you spend $40,000 a year and want a withdrawal rate of 3% you will need to save more than $1 million so it will take you longer to retire.
You can read more about withdrawal rates here:
http://blog.networthify.com/withdrawalrates/