A plaster mould works because plaster absorbs water. Press clay into it or pour slip into it, and the plaster draws water out of the surface in contact with it, so the clay firms and releases. That single property is why plaster and not any other material is used.
Why plaster and not something else
Absorbency. A resin, wood or metal mould holds clay against it and the clay sticks. Plaster pulls water from the contact surface, the clay shrinks fractionally away and the piece releases cleanly. That is also why a mould must be dry before use: a saturated mould absorbs nothing and the clay will not let go.
Press moulds and casting moulds
A press mould takes a slab pressed into or over it and is usually a single piece, used for dishes, tiles and repeated forms. A casting mould is closed, takes liquid slip, and needs to come apart in two or more parts with keys so it registers correctly. The second is more work to make and produces hollow forms nothing else will.
Making one
Build a wall around the model, mix plaster into water rather than water into plaster, let it slake without stirring, mix without whipping air in, and pour steadily from one point so air is pushed ahead of the plaster. It sets in minutes and gives off heat as it does. Let it dry for days before use, because a damp mould does not work.
Spelling, and what it means here
Mould is British and mold American, and both spellings are searched heavily in the UK because equipment, videos and books come from both. This site carries both rather than choosing, which is the same policy it applies to calipers and callipers and to modelling and modeling.
Questions people ask about plaster mould
Why are pottery moulds made of plaster?
Because plaster absorbs water from the clay in contact with it, so the piece firms and releases cleanly.
How long must a new mould dry?
Days rather than hours. A damp mould absorbs nothing and the clay will not release.
Mould or mold?
The same thing. Mould is British spelling and mold American; both are widely searched in the UK.