In German, you can add a prefix to a simple verb, forming a verb with a new meaning.

Prefix + verb = new verb

um + ziehen = umziehen

aus + räumen = ausräumen

Many of these new verbs are separable. This means that for a simple declarative sentence in the present tense, the original simple verb is conjugated and in the second position in the sentence. The prefix is at the end of the sentence. 

um|ziehen:
Meine Tante zieht am Samstag um.

In combination with a modal verb, the separable verb is at the end of the sentence in the infinitive. In this case, the prefix is not separated.

aus|räumen:
Ich muss meine Wohnung heute komplett ausräumen.

In the present perfect, a past participle is formed from the separable verb by placing -ge- between the prefix and the verb stem.

vorbei|kommen:
Du bist schon lange nicht mehr auf einen Kaffee vorbeigekommen.

You can often tell whether a verb is separable from its prefix.

• Verbs with the prefixes ab-, an-, auf-, aus-, ein-, mit-, nach-, weg-, zu- are always separable.
• By contrast, verbs with the prefixes be-, ent-, er-, ver-, zer- are not separable.
• Verbs with the prefixes durch-, über-, um-, unter- can be separable or inseparable.

You can also recognize separable verbs by their pronunciation. With separable verbs, the prefix is always stressed.

einziehen: separable
unterschreiben: inseparable