used to indicate the earliest possible time (if all goes well) or to mean 'if things go well' (e.g., 'at the earliest, it will arrive tomorrow'). It is the provisional form of the adjective 早い.
早ければ明日には荷物が届くそうです。
If everything goes well, the package should arrive as early as tomorrow.
早ければ来週中に返事をもらえると思います。
If things go well, I expect to get a reply sometime next week.
早ければいいが、工事が長引くかもしれない。
I hope it will be soon, but the construction may take longer.