Harness
Is for more experienced windsurfers, the help windsurfers speed and endurance, it is used when sailing upwind and unless you are experienced using in gusty winds is not recommended.
Harnesses fit around the chest, waist or seat.
The seat harness is the most comfortable and allows you to use all of your body weight to balance the rig, but for beginners chest harness are probably the best option.
Harnesses come in various sizes and forms, a well fitting harness should feel comfortable with no looseness but also not so tight it will rub.
Leg straps should be tightened first, then the spreader bar, and looseness will make hooking in and out harder.