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People often buy tuneable bodhráns without knowing the advantages of being able to adjust the tension of the skin. Obviously they learn eventually but if you're not sure, have a look.
The goatskin is susceptible to humidity and as such the tension can vary. If the humidity is too high the skin gets loose. If the humidity is too low the skin becomes too dry and gets tight. The tuning system eliminates these problems. If in a session you find your drum is too loud, you can let off the tension by loosening the screws. Alternatively if the skin is floppy, you can tighten the screws.
You can change the tension for different tunes. You may wish to higher the tension so as to be heard, or lower the tension so as to be humble (!).
I usually change the tension by turning the screws a quarter turn each, and then sound the bodhrán. If I am happy with the sound I leave it and play on. If I am not happy with the sound, I turn the screws again until I have found the sound I wish to achieve. Try raising and lowering the tension to hear the different sounds your bodhrán can make. It is favoured among bodhrán players to higher the tension if playing with a tipper at the higher end of the bodhrán, this is called high end playing where you're using the bottom end of the tipper only and playing the top half of the bodhrán. For this type of playing you need the tuning system to tighten the skin.