Hey,i really don't understand the "invisible" mode.

"/mode {yournick} +i makes yourself invisible to anybody that does not know your nickname"

What that means?It's very unclear to me.

You can use the /who command to /who a channel. You can do this from outside the channel before you join it, this worked more in days gone past. Invisible mode means you won't show up in that /who request. Some spam bots use /who to get nicknames without joining a channel, although it's quite an outdated method (because so many people use +i).