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Highly specialized eyewear designed to protect the eyes from the extreme intensity of solar radiation when directly viewing the sun or a solar eclipse. They are mandatory for safe viewing of the partial phases of an eclipse.
read more about solar eclipse glasses ...Must meet the international safety standard ISO 12312-2. These filters are thousands of times darker than regular sunglasses.
They filter out nearly all visible light, but critically, they must block $99.999\%$ of intense visible light and $100\%$ of UV and IR (infrared) radiation.
Only safe to use during the partial phases of a solar eclipse. They must be removed immediately if the eclipse reaches totality (only when the moon completely blocks the sun's disk).
No. Stacking multiple pairs of regular sunglasses is not safe. They do not block the dangerous infrared and high-intensity visible light adequately.
A safe solar filter typically has an optical density of 5 or greater, meaning they transmit less than $0.00001$ of the sun's brightness.
The main danger is Solar Retinopathy, permanent central vision damage to the macula from thermal and photochemical burn.