A harness consists of a wide belt and a hook which allows you to hook in at the harness lines attached to your boom. The power of the sail is controlled by your whole body weight, not only by your arms anymore which allows you to surf much longer and with bigger sails, without getting exhausted. The required technique can be learned quickly in every VDWS- windsurf center.
You have to choose between seat- and waist harnesses. Waist harnesses are worn a bit higher and have a better flexibility. Because of that they are used for wavesailing and freestyle. As a slightly advanced sailor you might appreciate – because of the higher hook position – the ease of hooking in.
Seat harnesses are worn lower (around your pelvis) and provide a better back support.
In the end it is a matter of taste which harness type you choose.