jQuery UI API - Easings
Easing functions specify the speed at which an animation progresses at different points. jQuery core comes with two easings: linear, where the animation progresses at a constant speed throughout; and swing (the default easing in jQuery core), where the speed is slightly slower at the beginning and end than in the middle of the animation. jQuery UI provides additional easing functions, ranging from variations in swing behavior to custom effects like bouncing.
Some easings can produce negative values during the animation. Depending on the animated property, the actual value might be zero. For example, you can have a negative left value, but you cannot have a negative height or opacity.
To better understand how an easing affects an animation, it is helpful to study the equation graphs. Below are the graphs of all available animations in jQuery UI.