The basics are the same as any other cartel. OPEC states cover enough of the supply-side of the market to be able to keep prices artificially high.
UAE leaving means UAE can price below OPEC's target and take more of the market. OPEC will have to react and lower prices or concede some of the market.
Does any of this matter if the major players can't ship oil through Hormuz? Who knows...
The UAE is trying to expand its ability to ship oil through Fujairah, so this could potentially undermine both KSA/Iraq and Iran.
OPEC was never a very effective cartel in the first place. Many of the members routinely exceeded production targets. And for geological reasons it's not like most oil wells can even be throttled down.