Leeches are cards that you keep on forgetting. Because they require so many reviews, they take up a lot more of your time than other cards.
Anki can help you identify leeches. Each time a review card 'lapses' (is failed while it is review mode), a counter is increased. When that counter reaches 8, the note is tagged as a leech, and the card is suspended. The threshold, and whether to suspend or not, can be adjusted in the deck options.
Anki will continue to issue leech warnings periodically for a difficult card. The warning interval is half the initial leech threshold. That is, if you have Anki configured to warn at 8 lapses, future warnings will happen every 4 lapses. (12, 16, etc)
Once a leech is found, there are a number of ways you can handle it.
Some leeches are caused by 'interference'. For example, an English learner may have recently learnt the words "disappoint" and "disappear". As they look similar, the learner may find themselves confusing the two when trying to answer. In these situations, it’s often productive to concentrate on just one idea. When that idea is firmly ingrained in your mind, you can then return to learning the other idea. So in these situations, you may want to leave one of the words suspended until you have learnt the other one well, and then unsuspend it in the browser.
Another way to manage leeches is to delete them. Consider if the material you’re struggling with is important enough to make it worth your while. By selectively deleting difficult and obscure items, you can dedicate more time to learning other material, and studying becomes a lot more fun.
Another approach is to change the way the information is presented. Perhaps the cards you have created have too much information on them, or perhaps you’re trying to memorize something without fully understanding it. Sometimes spending some time changing the way the card is phrased can help. It’s also a good time to think about making a mnemonic to help you remember.