Becky,
A calcium reactor is nothing more than a tank that holds media and water. The water is circulated by a pump usually pretty low flow unless its fluidized like the Deltec. Water is drawn in from your sump and co2 is mixed with the water. This lowers the ph below 7.0, the result of the lower ph disolves the media. The result is dripped back into your tank replentishing your ca and alk along with other trace elements that happen to be in the media.
The solenoid and regulator control the flow of co2 to the reactor. Most regulators have two gauges. One tells you how much pressure is remaining in the bottle. The other is the pressure leaving the bottle. The solenoid is just a electronic valve that will shut down the flow of co to the reactor if either you have a power failure or a ph controller switches off due to low ph.
Hope this is what you needed
Don