What are shelves in Tableau?

Shelves are the named drop targets around the view where you place fields to build it.

Columns and Rows define the structure — a field on Columns spreads data horizontally, one on Rows stacks it vertically. Between them they determine what kind of chart you get.

Filters restricts which data appears. Pages breaks the view into a sequence you can step through. The Marks card targets — Colour, Size, Label, Detail, Tooltip — are shelves too, controlling appearance rather than structure.

Some shelves appear only for particular mark types, such as Angle for a pie or Path for a line map.

The mental model worth stating: shelves for structure, Marks card for appearance.