When to use стол, and when столом? What’s the difference?
I don’t know if you’re familiar with grammatical cases but they actually do make a lot of sense and are absolutely needed because of the rather free word order and absence of articles in the Russian language.
столом is in the instrumental case. I’m guessing the context was “za stolom” = behind the table literally although in this context it means that the person is sitting at the table.