J interfaces with the environment are provided by the conjunction!: (foreign). In most cases the foreign result is a verb that provides a specific service. The left argument of foreign selects a general family of services and the right argument selects a specific service. The families of services selected by the left argument are:
2 host commands
9 global parameters
15 dynamic link library (shared library)
128 numerical functions
The foreign conjunction is documented in the J Dictionary.
You have run across a few specific uses of the foreign conjunction in earlier sections.
The derived verb 9!:11 was used in the Print precision section. The 9 selects the global parameters family and the 11 gives a derived verb that sets the print precision.
The load verb uses 0!:0 to load scripts. The left argument selects the scripts family. Look up the definition. The 0 right argument is treated as three decimal digits 000 and therefore executes sentences from a file, stops on an error, and is silent (does not display sentences or results).