A crisis is when you get a worsening of your symptoms. Either an increase of the ones you already have and/or new ones. This can happen when your body is detoxing and if the symptoms get intolerable, then you are detoxing faster than your body can keep up with.

This is what can happen when you take chelation for mercury. You can take it and then you can get hit with a healing crisis, which may mean you need to either lower the dose or stop and take a break before resuming on a lower dose.

Often symptoms are to be expected when your body is attempting to heal itself, but to try and keep those symptoms as tolerable as possible is the key. Not always possible, because the more poisoned a person is, the worse the symptoms they get during detox.