Topic, Subject
noun, の adjective
The topic that we are talking about is the topic that we are talking about. It's the subject of our conversation.
わだい
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
次の話題はこちらです。
The next topic is this one.
友だちと話題のフグをかいに行きました。
We went to buy the much-talked-about fugu.
それ、SNSでめっちゃ話題になってたよね。
That was trending a lot on social media, wasn't it?