Plato's Cursed Triangle also known as The Impossible Triangle is an optical illusion.
This explanation is written in "plain English" since the geometric equasions behind this illusion are too complex to be understood by anyone but math majors. ;o)
Simply put: The triangle at the bottom is not really a triangle at all, it just looks like one. It's a fake triangle.
The dark green and the red triangle in the big fake triangle seem to form a straight diagonal line from the lower left corner to the upper right corner, like they do in the real triangle on top. But in reality it's not a straight line in the fake triangle.
Compare the real triangle on top with the fake triangle at the bottom. Take a close look at their upper diagonal sides and you will see that the way they cross the white grid in the background is slightly different.
There is an invisible dent where the dark green and the red triangle meet in the fake triangle. As strange as it may seem, the slight dent is enough to throw off the straight line, and enough to amount to the volume of the missing square.