A clause is a self-contained grammatical package, like a sentence, but which is joined to one or more other clauses to form a sentence. Each clause is essentially an easily-identifiable chunk of a whole sentence which you could lift out and would not be gibberish by itself. For example:

I went to see the doctor because I had a severe eyebrow disorder. (2 clauses – ‘I went to see the doctor’ and ‘I had a severe eyebrow disorder’)

Jimmy had better pay his rent or Bulldog will come and smash up his 3D-TV.

Can you identify the clauses here?

And, in German:

Ich gehe jetzt zum Bahnhof und kaufe da meine Fahrkarte nach Frankfurt, wo das Festival stattfindet. (3 clauses – ‘Ich gehe jetzt zum Bahnhof’, ‘[ich] kaufe da meine Fahrkarte nach Frankfurt’ and one relative clause, ‘…wo das Festival stattfindet.’)