I think I understood the basic contents of the seminar today.
I got the gist of what they were talking about in the seminar today.
ーI think I understood the basic contents of the seminar today.
「今日のセミナーの基本的な内容を理解したと思う」
the basic contents of the seminar「セミナーの基本的な内容」
ーI got the gist of what they were talking about in the seminar today.
「今日のセミナーで話していたことの要点はわかった」
the gist of what they were talking about in the seminar「セミナーで話していたことの要点」
逆に全然分からなかった場合は、次のように言うと良いでしょう。
ーI had no idea what they were talking about in the seminar today.
「今日のセミナーで何を話していたのか全く分からなかった」
ーI didn't understand what they were talking about at all in the seminar today.
「今日のせいなーで何を話していたのか全然理解できなかった」
ご参考まで！
"I think I have a general understanding of what today's seminar was about."
- "I think I have a general understanding of what today's seminar was about."
"I think ~" 「〜と思う」
"I have a general understanding" 「だいたい理解できた」
"what today's seminar was about" 「今日のセミナーの内容」
カナダ 
オーストラリア 
