Replacement cost is what it would cost to completely replace your house or any part of it, or replace your belongings with new items of "like kind and quality" without deduction for depreciation. Rebuilding your home could cost more or less than the "market value" of your house.
Insurance policies often restrict the amount of replacement coverage. Make sure you understand the maximum limit on your replacement cost coverage.
Actual Cash Value (ACV) is the cost to repair or replace the damaged property with materials of like kind and quality, less depreciation of the damaged property. This generally applies to your "contents" coverage but there are policies that pay ACV on dwellings, as well.