What is Flagellated?

According to a new book, Pope John Paul flagellated himself regularly to emulate Christ's wounds and signed a secret document saying that would resign instead of ruling for life if he became incurably ill.
I don't know what the word "flagellated" means, is it some sort of deep wound?
How do you flagellate yourself, using a knife, or nailing to hand to something?
Please help me understand the meaning and definition of the word flagellate.

The book is called Why A Saint and is written by Monsignor Slawomir Oder.
Flagellated is what you call small cells that have a "tail" or something that resembles a whip.
However what the pope did to himself is another kind of flagellation.
From the bible, flagellation is defined as the process of driving or punishing someone, typically through whipping.

Pope John Paul practiced flagellation and flagellated himself to emulate Jesus Christ's. The pope was a bit crazy.
Some will say the pope was some sort of emo.