What is the meaning of Ad_lib?

At pleasure.

At will.

As much as desired, to one's fill, without restriction.

Extemporaneously.

Extemporaneous, impromptu.

An instance of something improvised, especially a line.

Improvised vocals added after the main vocals have been recorded which serve to emphasize the rhythm or fill the lyrical bar line.

To perform without a script.

To perform without preparation.

To perform extemporaneously.

To perform ad libs; to improvise lyrics after the main lyrics have been recorded.

To perform (lyrics) as ad libs.

Source: wiktionary.org