J-Link OB is an on-board debug probe as part of YOUR eval board. The microcontroller has a USB-interface on one side and a JTAG/SWD + SWO etc. interface on the device side. It allows debugging the target device on the board it comes with, without the need for an additional debug probe. J-Link OB is used on various eval boards by several eval board manufacturers for various target CPUs.
Having a J-Link right on the eval board allows customers to fully use and test the target CPU, providing full debug capabilities. At the same time, the eval board can remain small and inexpensive. All features of the regular J-Link can be used, including download into Flash memory.
Example implementations that support Drag-And-Drop flash programming
The following boards are already using J-Link OB implementations that support Drag-And-Drop flash programming:
NordicSemi nRF51-DK (https://www.nordicsemi.com/eng/Products/nRF51-DK)
https://www.segger.com/products/debug-probes/j-link/models/j-link-ob/