The "orchadmin" command is documented. the "dump" option does exactly that, it dumps the contents of the DataSet to standard output. This command doesn't modify the dataset at all, just does the DataStage equivalent of a "cat" of the contents.
You asked two UNIX questions;
2 is stderr
/dev/null is a special UNIX file or null device. It discards all data written to it. In this case it is getting rid of any errors being written by the previous command.