I assume that they are talking about easing gradients, which is interpolating the gradient’s color stops using a cubic bézier easing function [1]. It gets rid of the sharp edge you sometimes see in linear gradients. Think of CSS animations and their easing-functions.

There are various Sketch and Figma plugins which let you modify the easing function and generate the CSS for it.

[1] https://css-tricks.com/easing-linear-gradients/