Reduced oxygen saturation in blood is called hypoxia (though strictly that means reduced oxygen in the tissues). If there is marked reduction in oxygen saturation of blood, it may lead to bluish discoloration of skin and tongue, called cyanosis.
Mildly reduced arterial oxygen saturation may be due to mild respiratory or cardiac diseases. A common lung condition is chronic obstructive airway disease (or chronic bronchitis, most commonly due to smoking). Congenital cyanotic heart diseases like Tetralogy of Fallot are generally discovered and treated in childhood. Arterio-venous malformations in lung is another cause of arterial oxygen desaturation which may be present in otherwise healthy persons. Persons staying at high altitudes have reduced blood oxygen saturation due to low atmospheric pressure leading to reduced partial pressure of oxygen in inhaled air. Sleep apnea syndrome often seen in obese persons may also lead to hypoxia. Other causes include neurological diseases leading to respiratory depression, respiratory conditions such as severe pneumonia, pulmonary embolism (clots in arteries supplying blood to the lungs), pulmonary edema (lung congestion), pulmonary fibrosis etc, but in all these the person is obviously ill.
In an apparently healthy person, mild hypoxia is likely to be due to smoking or pollution related chronic obstructive airway (pulmonary) disease (COAD or COPD), pulmonary arteriovenous malformation or early pulmonary fibrosis.
Mild hypoxia may not result in any symptoms. It may lead to increased hemoglobin level in the blood to compensate for reduced oxygen saturation, as commonly occurs in persons living at high altitudes. The main worry is that the underlying process may progress and hence the cause of desaturation should be investigated.