He means jumping and landing on a box or obstacle rather than jumping right over it and landing on the other side.
Everyone has their limits with this and you will amaze yourself with what you can do, if you have the confidence.
The hardest thing is the fact you're landing on it. If at all possible I would find something which you can hold on to once you're up there. Bit hard to explain but jumping onto something and then having something to grab for safety is a lot easier than jumping onto something and having nothing. Once you know what height you can reach then you can start doing it without grabbing something.
Plenty of stuff like this in Cardiff Bay