Kind of like "Do something vague (but specific) with your arms".
"um" has a very vague meaning - it will have a different meaning in different words.
So you essentially have to learn -um- words as a whole -- that "brakumi" means "to hug" but "manumo" is a cuff (at the end of a shirt sleeve) and not "a hug with your hands" or "a clap" or "wringing your hands" or anything like that.
- Have a look at https://en.wikipedia.org/wiki/Esperanto_words_with_the_suffix_-um
- a shorter list is at https://en.wiktionary.org/wiki/-um-
- that links to a list of words, but without translations: https://en.wiktionary.org/wiki/Category:Esperanto_words_suffixed_with_-um-
- this may be useful, too: http://bertilow.com/pmeg/vortfarado/afiksoj/sufiksoj/um.html