It's really difficult to explain prepositions, or to translate them between languages. How would you explain someone when to use "to" in English? There are so many different uses. It's the same with with "um". The general idea is "around, about, encircling something, on the circumference around something". This idea is even stronger when used in the circumposition "um X herum". But this meaning has shifted when referring to a time, "um 4 Uhr" no longer means "around 4 o'clock" ("gegen 4 Uhr") but "at 4 o'clock". And "um zu + Infinitiv" means "in order to + infinitive". I guess there are more uses I can't think of at the moment, you just have to consider them on a case-by-case basis. If you can some concrete example where the meaning is baffling you, maybe we can explain those.