It defines the value of the standard deviation to the Gaussian function, i.e., how many pixels on the screen blend into each other; thus, a larger value will create more blur. A value of 0 leaves the input unchanged. The initial value for interpolation is 0.
https://developer.mozilla.org/en-US/docs/Web/CSS/filter-function/blur
https://developer.mozilla.org/en-US/docs/Web/CSS/filter-function/blur