。
Practice makes perfect.
Literal
Practice [subject-が] perfection [object-を] produces.
A direct Japanese rendering of the English proverb. 完成 here is used as an object ('the completed state, perfection'). 生む means 'to give birth to, produce.' Notice how the particles mark a clean cause-effect: 練習が (subject) → 完成を (result) → 生む (produce).