To open a lid; to lift the cover of a container (literal action).
鍋の蓋を開けると湯気が上がった。
When I opened the pot's lid, steam rose.
缶の蓋を開けるのに栓抜きを使った。
I used a can opener to open the can's lid.
ふたを開ける前に中身の温度を確かめてください。
Please check the temperature of the contents before opening the lid.
To make something public or reveal information (to 'open the lid' on something), i.e., to disclose previously hidden facts.
調査報告書が出て、ついに会社の不正の蓋を開けることになった。
The investigation report came out, and finally the company's wrongdoing was exposed.
今回の証言で事件の蓋を開けるような新事実が出てきた。
This testimony revealed new facts that opened up the case.
To start something — an idiomatic expression for beginning an event or activity.
夏祭りは夜八時に蓋を開ける予定だ。
The summer festival is scheduled to start (open) at 8 PM.
新しい店が今月末に蓋を開けるらしい。
It seems the new shop will open at the end of this month.
To look at the results or see what happens (idiomatically, as in 'when we opened the lid, ...' implying checking outcomes).
計画を実行して蓋を開けてみたら、予想以上に利益が出た。
We carried out the plan, and when we saw the results, it made more profit than expected.
新システムを導入して蓋を開けてみると、問題がいくつか見つかった。
After introducing the new system and checking it, several problems were found.
Used to mean the opening of a theater or venue (to open the house / begin a performance).
開場時間になり、劇場が蓋を開けると観客が続々と入ってきた。
At opening time, the theater opened its doors and audience members streamed in.