These are dentures that are fitted immediately after a tooth or several teeth are extracted. They are a temporary denture, used to replace these teeth as the mouth heals. This process can take several months, so the final denture/bridge etc. is not fitted until the healing is complete.
In the interim time the immediate denture acts as a ‘stop-gap’. Acrylic (plastic) is normally used to make this type of denture. Acrylic can be easily adjusted and remolded as the gum shrinks away after extraction.