This is the 748th most frequent Polish word.
znowu
"Znowu" means "again" in English.
In this sentence, 'znowu' is used to indicate that the action of raining is happening once more, after it has previously occurred.
Znowu pada deszcz.
It's raining again.
In this context, 'znowu' highlights the repetition of the action, indicating that forgetting about the meeting has happened multiple times.
Znowu oni zapomnieli o spotkaniu.
They forgot about the meeting again.
Here, 'znowu' expresses the return of the state of being happy, which implies that the speaker was happy at some earlier point, and that feeling has now returned.