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.
If possible, please reply by tomorrow.
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'.