^ yup. The needle valve gives fine control of the number of bubbles you're injecting into your tank. You look at the bubble counter when you adjust the needle valve to see how many bps (bubbles per second) you're putting into the tank. This isn't the best method to see how much CO2 is in your tank, however, due to differences in diffusion methods and differences in outgassing of CO2 due to varying equipment.A drop checker gives you the level of co2 in your tank.
A bubble counter shows how much co2 you are injecting in your tank.
A regulator is a device attached to a co2 tank that regulates the co2pressure coming out of the co2 tank,so you need it in order to decrease the high pressure(800-1200 psi) to 20-30 psi,then a needle valve decreases it even further to .5 psi ,this is what you'll see in a bubble counter.