For most people (hip pads, braces or cheek plates) just keep you from sliding from side to side when you lean your boat. Many commercial glass kayaks actually have the glass seat hung on 4" wide struts or hangers that attach to the deck. People pad these or just use them as is.

You just want padding on each side of your hips to keep you in place on your seat. Not too tight - just there to prevent sliding side to side. It's personal call but most manufacutturers have them where the hip area is when seated.