A standard for transferring media files between two connected devices. Typically, the files are photo (or similar) files and the connection is USB.
A typical use would be transferring photos from a phone over a USB cable.
Unlike some other protocols, PTP is designed to allow multiple devices to access the same set of files at the same time, without the risk of files becoming damaged.
For example, consider photo files stored on a phone's internal memory, and that phone connected to a computer. With a protocol such as USB Mass Storage, the portion of internal memory with the photos must be disconnected (in software) from the phone before the computer can access that memory to add or remove photos. Otherwise files could be damaged if both devices attempt to change them at the same time. It isn't possible to disconnect that internal memory if the phone has one pool of internal memory that also stores the software that keeps the phone running (as some phones do.)
PTP solves that problem by carefully managing the connection and access to files, ensuring that files aren't damaged. The drawback is that MTP only allows simple access to certain types of files, whereas other protocols allow access to more files in more parts of memory.
PTP is almost identical to MTP, a related protocol for other types of media.