Sturdy foam, with zippers, so I imagine the covers can be washed. Ties are firmly attached, and long enough for a good wrap to secure the cushion to the seat. These are thin, but QUITE firm.
I think they will work well to add support so my seat and back will hurt less after long dinners and games at the table.
They cover a fairly large space on my chairs, and don’t adjust much, so make sure your chair seat is shaped and sized correctly.