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This question already has an answer here:

Explain me please why do i have these very very small points (like pixels) after rendering. Is it connected with lightning and how to fix it?enter image description here

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marked as duplicate by David Apr 23 '17 at 22:59

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Those are called fireflies and are caused by not enough samples.

I will refer you to the manual:

https://docs.blender.org/manual/en/dev/render/cycles/optimizations/reducing_noise.html

To understand where noise can come from, take for example this scene. When we trace a light ray into the specified location, this is what the diffuse shader “sees”. To find the light that is reflected from this surface, we need to find the average color from all these pixels. Note the glossy highlight on the sphere, and the bright spot the lamp casts on the nearby wall. These hotspots are 100x brighter than other parts of the image and will contribute significantly to the lighting of this pixel...

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