An expression meaning 'as soon as something is finished/ready'. Often used in the pattern '...でき次第, ...'.
商品が出来次第、発送いたしますので少々お待ちください。
We will ship the product as soon as it's ready, so please wait a moment.
書類ができ次第、メールでお送りします。
I'll send the documents by email as soon as they're done.
時間ができ次第、手伝いに行きます。
I'll come to help as soon as I have time.