Some verbs beginning with the prefixes durch-, hinter-, über-, unter-, wider- take -ge-, -zu- between the prefix and the rest of the word, others are inseparable. The verb is separated if the prefix is stressed.
Usually, in a literal, concrete sense, these verbs are separated: er setzt über (takes to the other side), er übersetzt (translates).
In the case of the prefix um- we have a clear semantic distinction: accented um in a separable verb means a change, transfer of the object, unstressed um- means “around”, surrounding:
ich stelle den Tisch um - I move the table
wir umstellten as Haus - we've surrounded the house.