Ducking is an effect often used by DJs and in all kinds of broadcasting. It’s often achieved by the use of side-chain compression. Side-chaining is when you use the dynamic levels of one source to control the compression of your signal. It’s best described in example, for instance, it’s common to use this for the bass and kick drum. Compressing (thus lowering the levels) the bass when the kick drum enters to have them both fit together.