The Gait Designer (get it here) is a Java application that allows you to create a number of "animations" - say: walk, crouch, kneel etc.
Each animation can have a number of key frames for each limb (ie servo/joint) and the Gait Designer produces a smooth transition between these key frames (so called 'in-between-ing').
Whilst designing the gait you can connect your computer to a micro-controller running WebbotLib so that the servos move to the selected position for each key frame. Just 'drag' the servo position 'on-screen' and the robot limb will move in unison!
Whilst designing the gait you can, at any time, press the 'Play' button to animate the gait. The 'on-screen' sliders will move in real time, and if you are 'Connected' to the robot, then the robot limbs will move as well.
Once completed you can export the gait into a file that can be included from your WebbotLib project. You can then play any of the animations, at any speed, without requiring any form of connection back to your PC.
Some people use it for walking gaits - others use it for other purposes:-