An injection mould tool is the tool that a material is injected into to make an injection moulding. Typically this is a thermoplastic although other materials can be formed using injection moulding.

The rest of my answer relates to plastic injection moulds as this is my area of expertise. Mould tools are pretty costly precision machined items. As well as machining out the form of the component that is required (with a shrinkage factor as plastic shrinks after it is injected into the tool) they also have to include some form of ejection system to enable the part to be removed from the mould (typically an arrangement of pins that are moved by a hydraulic action on the injection moulding machine) and some method of heating/cooling to control the temperature of the mould (typically drilled channels in the tool to allow water or oil at a preset temperature to pass through the tool although electric heaters can be used (an inferior system in my opinion but can be cheaper))

Depending on the geometry of the part to be made tools can incorporate various "actions" which are basically bits of tool that move either before, during or after the tool opens. These can either be powered by hydraulics or by the movement of the tool as it opens.

Tools can be made from Aluminium for low volume or prototype moulds, pre-toughened steel (P20) for mid volume (up to a few million parts)or hardened steel. In reality the choice of tool material is largely dictated by the choice of plastic, glass filled materials and high temperature materials tend to be very aggressive and a P20 tool may only last for a few hundred parts with the most aggressive materials.