What is oxygen saturation?

If one of the four heme groups is carrying an oxygen molecule, then the haemoglobin molecule could be said to be 25% (1/4) saturated with oxygen.

If all four are combined with oxygen then it is 100% saturated and cannot carry any more - it can't be more than 100% saturated.

On a larger scale, 25,000,000 million haemoglobin molecules will have 100,000,000 oxygen carrying haemo groups. If 95,000,000 of these are carrying oxygen then that sample of haemoglobin is 95% saturated.

This is oxygen saturation - which is what a pulse oximeter measures - see the pulse oximetry section of this site.