This is the 180th most frequent German word.
dass
"That" (used as a conjunction to introduce a subordinate clause).
Here, 'dass' introduces a subordinate clause, serving as a conjunction that links the dependent clause to the main clause, indicating the object of the verb 'weiß' (know).
Ich weiß, dass du die Wahrheit sagst.
I know that you are telling the truth.
In this sentence, 'dass' introduces a subordinate clause, specifying the content or the detail that emphasizes the adjective 'wichtig' (important).
Es ist wichtig, dass wir pünktlich sind.
It is important that we are on time.
Again, 'dass' functions as a conjunction linking the subordinate clause to the verb 'glaubt' (believes), indicating the content of her belief.