The state of something being carried out or in operation right now; commonly used on signs to mean 'in progress' or 'now running'.
現在、メンテナンス実施中のためアクセスできません。
The site is currently inaccessible due to maintenance in progress.
キャンペーンは今月末まで実施中ですので、ぜひご利用ください。
The campaign is running until the end of this month, so please take advantage of it.
工事実施中のため、前の通りは通行止めになっています。
Due to construction work in progress, the street ahead is closed to traffic.