Rapid prototyping (RP) refers to a set of production processes that can generate one-off custom plastic and metal parts. 3D printing or SLA are common RP techniques for plastic parts. 3D printing can work for metal parts also, via a few different techniques. RP is great for making small quantities of parts, either for design iterations, or to make a one-of-a-kind part or an intermediate work product that leads to a part (e.g. a 3d printed pattern for a lost-wax jewelry production process), or to make fixtures. RP's strength is versatility and speed for making 1 of something.
RP techniques are very versatile and a huge boon to the product design and development sector. They are not all there in terms of fit/finish, material properties, and speed and cost effectiveness where it comes to making a lot of parts (such as in a mass production scenario).
Mass production commonly refers to the stage of product development and manufacturing where a product team is ready to commit a design to production. Example mass production processes include injection molding for plastic parts and investment casting / die cutting and so forth for metal parts. These processes are very expensive and slow to set up as it involves "tooling" - but once set up, you can create hundreds of thousands of the exact same part with the same quality in a short time, with a low part cost, none of which is true for RP processes.
As of 2020, there remains a line between RP and mass production. Generally, RP remains good for making 50 or less of something, whereas mass production is good for making hundreds of metal parts and tens of hundreds of plastic parts. There is a gray area between those quantities that are difficult to bridge and people's mileage may vary. Hardware entrepreneurs will need to look at the specific needs of their project and the stage of play or their venture to decide which way to go for their next production round. This is a very important decision, as mechanical designers need to take the production techniques and processes into consideration.
Having said that, additive manufacturing is a rapidly evolving field. We already see advances in the use of additive manufacturing processes for very specialized production processes, such as GE's use of metal 3d printing to make fuel nozzles. We are excited to see what the next decade will bring.