Detailed
Expert
い adjective
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of detailed? It's also detailed.
When something is detailed it is made up of a huge quantity (くわ) of little things. And if you're an expert on something, you'll have a huge quantity of knowledge, too.
ずいぶんシメサバについて詳しいんですね。
You are quite the expert on mackerel sushi.
あの人はコンピューターについて詳しそうだ。
That person looks like they know a lot about computers.
おお、マジかよ？ 詳しく教えてくれよ。
Wow! Really? Tell me everything.