These may be used by trans women and non-binary people who were assigned male at birth.
They are silicone breasts which can be placed in a bra cup or you can purchase bras with special pockets to hold the forms more securely.
They are also used by women who have undergone mastectomy surgery and may be advertised as such. Any soft filler item (such as socks, tissue, tights etc) can also be used to pad out a bra though it is less likely to create a good shape.