-um is a “non-precise” suffix that broadens or gives a different meaning to the root. Generally, you have to learn each um-construction as a separate word. And while some new um-words are occasionally created (like retumi, “to browse the internet”), it’s not really an actively productive suffix to the same degree that others are.

However, some um-words fit into categories. For example, some of them name articles of clothing:

• kalkanumo = heel (of a shoe)
• kolumo = collar (of a garment)
• nazumo = pince-nez (old-fashioned eyeglasses)
• plandumo = sole (of a shoe)

Some indicate actions done using a thing:

• akvumi = to water, to irrigate
• aerumi = to air out
• literumi = to spell using letters
• sapumi = to soap up

Some indicate actions involving a thing:

• foliumi = to leaf through (a book, etc.)
• brakumi = to hug
• okulumi = to make eyes at, to ogle

Lots of um-words, however, don’t fit in a category. Like laktumo (“milt,” based on lakto because of the milky appearance) and cerbumi (“to wrack one’s brain”).

PMEG (Plena Manlibro de Esperanta Gramatiko) has a fairly complete listing of um-words in current use.