I can't really grasp what this conversation is about.
I don't really understand what these two are talking about.
ご質問ありがとうございます。
① "I can't really grasp what this conversation is about."=「この会話が何についてかあまり良くつかめない。」
② "I don't really understand what these two are talking about."=「この二人が何について話しているのかあまりよく分からない。」
ご参考になると幸いです。