An idiomatic expression meaning to bring something to an end or close a matter; literally, to put a period at the end of a sentence.
彼らはプロジェクトにピリオドを打つことに決め、解散した。
They decided to put an end to the project and disbanded.
長い議論にようやくピリオドを打ち、次の議題に移った。
They finally put an end to the long discussion and moved on to the next agenda item.
この文はピリオドを打つべきか疑問に思ったが、結局句点を付けた。
I wondered whether to put a period at the end of this sentence, but in the end I added a full stop.