A plain stainless washer sized for 3/8 inch fasteners. Its job is to spread the clamping load over a wider area, which matters most on aluminium, thin sheet and composite panels where a bare bolt head or nut would dig in and eventually go loose as the material creeps. It also gives the fastener a clean, flat bearing face to turn against, so more of your torque goes into clamping force and less into friction and surface damage. Stainless keeps it from rusting and staining the panel around it in exposed or underbody positions. Use one under the nut as a minimum, and under the head too where the surface is soft or the hole is oversized.