Freelensing means you take photos with the lens detached from the camera but held in place and moved around to focus. This also lets extra light in sometimes causing light leaks and giving a vintage look and feel. You only need to move the lens a few millimetres (fractions of an inch), and doing it this way, there’s not much risk of dust getting in to the sensor. If you want lightleaks or the super-macro kind of effect, you will need to hold the lens a little bit further away (probably no more than a finger width, though). This is a bit more risky if you’re worried about dust, so try not to hold the lens away from the camera for too long and only do it in a dust-free environment.
Here’s an example by Luke Roberts:
(thank you Karin, via drs.ch)