It means if you start to execute the method, but change your mind, you can stop without much harm.
For instance, partial hanging is reversible because if you change your mind before you pass out, you can stop by getting out of the noose.
But a poison with no known antidote is not, because once you take it, you will die even if you suddenly change your mind.
So reversible means "I can start to kill myself, but can stop it halfway through if I change my mind and want to live."