A phrase used when giving reasons or points, meaning 'for one thing...' or 'firstly...'; often followed by a second point.
一つには予算が不足していること、二つには人手が足りないことが問題だ。
For one thing, there is a lack of budget; for another, there is insufficient manpower—those are the problems.
一つには安全性の確保が最優先だ。
For one thing, ensuring safety is the top priority.
彼が失敗したのは一つには準備不足のせいだろう。
One reason he failed is probably insufficient preparation.