In a nutshell, a “leech” is an item on Wanikani that you keep getting wrong. I assume they are named that way because they constantly leech away from your accuracy percentage.
You can try these two third party apps if you want: SRS Level Progress to monitor your leeches, and Leech Trainer to actively practice them.
Hope this helps! 
EDIT: Derp, I somehow responded to the wrong comment 
This is directed at @dpicardal