Consult the documentation for
myprog - whatever that is.
If whoever wrote it was kind enough to write a man page for it, then use
man myprog, or an info page - use
info myprog, otherwise read whatever documentation there is for
myprog.
-d is an option to the program, and
dictionary is an argument to that option. You're supposed to replace
dictionary with, I presume, the name of a dictionary.
I'm closing this thread. Please read the documentation for the program you are referring to. If you need help understanding the general concepts then consult our
man pages, which contain hundreds of commands, most of which take options just like "
myprog" - whatever that is - does.