In IDL you can have variables and keywords like this:
pro test, variable, keyword=keyword
you can then call the procedure test in the following ways:
test, 1: then 'variable' is set to 1 and keyword to 0 (i.e. not set)
test, 1, /keyword: then 'variable' and keyword are set to 1
test, 1, keyword=5: then 'variable' is set to 1 and keyword is set to 5