In an inline CO2 reactor, you basically inject CO2 into an enclosed space (typically a canister or tube) where there is a lot of turbulent flow (ie through bioballs) and the increased contact time usually results in 100% dissolution of the co2 into the water, and makes for a very efficient use of your CO2.
Something like this:
So the water from your canister filter enters through the bottom. It has to go through the bioballs and so has really turbulent flow, and spends longer in the chamber as a result. The CO2 is injected into the canister from the smaller barb on the left on top of the canister, where it travels down a tube to the bottom of the reactor and bubbles up to the top. Except that all the water in there, the turbulence and pressure, and it generally dissolves 100% into the water, and then leaves the reactor out the top, then is delivered into your tank. The small valve to the right on top is to purge gas when you're setting it up.