This is the path (bus) used to fetch data from memory to the processor. In some cases the internal and external data buses are the same bit-size but in others, the external data bus can be either narrower or wider. The external data path is normally not as wide as the internal data path.
Having a smaller external bus (data path) will slow the performance of the-CPU, but it makes it simpler to upgrade earlier system designs to a new processor. Having a larger external data bus will improve the performance as data can be brought into the CPU in large chunks.