Photosensitivity is any abnormal reaction of the skin when exposed to sunlight. It can be a symptom of several medical conditions (such as certain forms of porphyria) or a side effect of certain medications (often antibiotics). Although it can take many forms, it usually manifests similar to a serious sunburn.
If the condition is caused by a medical condition, avoidance of sunlight is required. This can include not being outdoors except when fully clothed (including a head covering) or using special filters indoors to block natural sunlight coming through windows. If it is a side effect, the medication should be immediately discontinued.