What is the meaning of Ad_lib?

At pleasure.

At will.

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

Extemporaneously.

Extemporaneous, impromptu.

That provides as much as each one desires, to each one's fill, without restriction.

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