A polite, common way to express a request or preference when something is not strictly required. Often used to soften suggestions or express a wish.
できれば、明日までに返事をください。
If possible, please reply by tomorrow.
できれば、もう少し静かにしてもらえますか。
If possible, could you be a little quieter?
よろしければ is a more formal, polite way to say 'if it's alright with you', while できれば is the everyday casual equivalent focusing on feasibility.
The conditional form of the verb できる (to be able to do, to be possible). Literally 'if it can be done'.