For a long time I didn't understand what teachers wanted me to do when they said kakari-geiko or ji-geiko. I even sat down with a 7th dan teacher and asked him to explain, but I think the language barrier meant I didn't fully understand. He ended up saying 'you'll learn'.
Now I've had the chance to read up on the terms, and ask around, and I think I understand:
Kakari-geiko is "charging practice" which means just charge at your opponent without stopping, without regard for personal safety and without concern for winning or losing. Winning and losing do not exist in kakari-geiko. You throw everything you have at your opponent as fast as you can until you fall over, or stop just before.
Ji-geiko is "fighting practice" which is where you take your time and utilise everything you have ever learnt to try to get an ippon. Don't use any trickery here, just try to do things as correctly as possible.