A steam accumulator is a device to store heat, so that it can be released as saturated steam when required.
During periods when then there is an excess of steam (ie periods of low demand) steam is directly injected into a large volume of water (typically held in a horizontal cylindrical presssure vessel - the accumulator). This steam condenses in the water, heats it up and holds it at the saturation pressure. During periods of high steam demand, the pressure of the vapour above the water surface in the accumulator decreases in pressure, as a response to this pressure drop vapour flashes from the water surface and saturated steam is released to make up for the high process steam demand. This process will continue until either the main steam supply is able to meet the process demand or until the pressure of the vapour above the water surface is the same as the process steam pressure and no more pressure drop is available.