What is meant by the phrase 'false tab'

'False tab' is the term used when a performer pretends to leave the stage to build expectation that the audience will require an encore.

'Tab' is derived from the word 'tabs' which is the name given to the stage curtains of a theatre.

A performer (such as a singer) may be ask by a stage director if they have a false tab in their performance when explaining their stage routine, often referred to as a set list. This will often be before their penultimate song(s) which would usually be the encore of the performance.