-d $FILE stands for "test to see if $FILE is a directory";

_ stands for "the topic (of the current conversation)"^*;

so, -d _ means "test to see if the last tested file is a directory";

^*:

$_ is the topic of the for, will be used by default by regexen and a lot of functions
@_ is the collection of topics for a function (the arguments!)
_ is the topic for file tests.