As much as possible; to the greatest extent one can—an adverbial phrase used to request or indicate doing something to the best of one's ability.
できるだけ早く来てください。
Please come as soon as possible.
できるだけ野菜を食べるようにしている。
I try to eat as many vegetables as possible.
できるだけシンプルに説明してください。
Please explain it as simply as possible.
遠すぎるときはできるだけ電車で行った方がいい。
When it's too far, it's best to go by train if possible.