Where to find it:

NodeGraph / Right Mouse Click / Add Nodes / Math / Extension Pack / S /

smoothstep performs smooth Hermite interpolation between 0 and 1 when edge0 < x < edge1. 

This is useful in cases where a threshold function with a smooth transition is desired. smoothstep is equivalent to:

    t = clamp((x - edge0) / (edge1 - edge0), 0.0, 1.0);

    return t * t * (3.0 - 2.0 * t);

Results are undefined if edge0 ≥ edge1.

https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/smoothstep.xhtml

•Edge 1

Specifies the value of the lower edge of the Hermite function

•Edge 2

Specifies the value of the upper edge of the Hermite function

•X

Specifies the source value for interpolation

•Output RGB

Separate computation for Input Channels Red, Green, Blue with an Alpha Channel of 1.0

•R

Computation for Input Red channel only, saved into R, G and B channel with an Alpha of 1.0

•G

Computation for Input Green channel only, saved into R, G and B channel with an Alpha of 1.0

•B

Computation for Input Blue channel only, saved into R, G and B channel with an Alpha of 1.0

•A

Computation for Input Alpha channel only, saved into R, G and B channel with an Alpha of 1.0