This is an entry level system that does not allow you to control the CO2 injection rate. If you want some degree of control then you need to upgrade to a system with a regulator valve and with a proper CO2 diffuser.
Flow rate measures water movement in liters/hour. It is used to measure the water circulation inside the tank (for example, the flow provided by a circulation pump), or the amount of water being circulated through the filters. In a tank with CO2 injection, you should aim at a flow rate of at least 5x the total water volume, ideally 10x. However, higher flow rates also increase surface agitation and gas exchange. So, if you increase the flow rate and keep using this CO2 system you might actually end up with less CO2 in the tank than you have now...