Wild camping is camping but not on a campsite. It could be with permission from a landowner in a field, or on a moor or in the mountains. You'll need to carry everything you need. The Walking Woman can provide basic equipment and advise you on what you might purchase.
Why Wild camp, apart from the smug sense of self sufficiency, the peace of the wilderness, the ability to access deep into the hills, I can't think why else I might want to wake up on the shore of a remote lochan, and have it all to myself.