The VISOR is a device that allows Geordi Laforge to see, in spite of him being blind from birth. It scans through the EM spectrum at a much greater range of frequencies than those that the normal human eye can detect. The device transmits the data through data ports implanted in the sides of his head.
In TNG "Encounter at Farpoint", Geordi said that using the device was painful, but the prescribed medical treatments would interfere with its function. This aspect of the device's operation was never addressed again in the series.
Like tricorders, the VISOR seems to run active scans at all times (TNG "Parallels"). This could make it a liability in situations where stealth is required.