What does 'lining' a painting mean?

Lining is a major treatment in paintings conservation, and should only be used when absolutely necessary

Elements of Painting: Line and Shape - FeltMagnet

Conserving Canvas Project - The Courtauld

The art of one line drawings. What is a one line drawing and what

Line as an art element (article)

How to Hang Lining Paper – A Step-by-Step Guide

Line in Art - Discover the Different Types of Line in Art

Our Guide To Jacket Linings - Fabric Types, Half vs Full Linings

Lining the Painting

80 Best Examples Of Line Drawing Art - Bored Art

Lining a painting on canvas - Romoe Network