As in English you can refer back to something that has already been mentioned by saying it.
In German you use a different word depending on whether the word is masculine, feminine or neuter.

With der words, ie. masculine words, you use er:

 Try guessing the right word before displaying the answer Hier ist der Ku’damm Here is the Ku’damm
Er ist hier It is here

With die words, ie. feminine words, you use sie:

 Try guessing the right word before displaying the answer

Hier ist die Kirche Here is the church
Sie ist hier It is here

With das words, ie. neuter words, you use the word es:

 Try guessing the right word before displaying the answer

Hier ist das Branderburger Tor Here is the Brandenburg Gate