CSS timing functions control the speed curve of an animation or transition.
linear: The animation or transition progresses at a constant speed.
ease: The animation or transition starts slowly, accelerates in the middle, and slows down at the end.
ease-in: The animation or transition starts slowly and accelerates gradually until the end.
ease-out: The animation or transition starts quickly and decelerates gradually until the end.
ease-in-out: The animation or transition starts slowly, accelerates in the middle, slows down, and then accelerates again to the end.
cubic-bezier(x1, y1, x2, y2): This function allows you to specify a custom speed curve. The x1 and y1 parameters control the start of the curve, the x2 and y2 parameters control the end of the curve, and the values between 0 and 1 control the slope of the curve.