What is marks card in Tableau?

The Marks card is the panel controlling how the marks in a view look.

The dropdown at the top sets the mark type — bar, line, area, square, circle, shape, text, map, pie, Gantt, polygon, density — or Automatic, which lets Tableau choose from the fields in play.

Below it sit the property targets. Colour splits marks into a colour legend or applies a gradient. Size scales them. Label prints values on them. Detail increases granularity without changing appearance. Tooltip controls hover text. Depending on mark type, Shape, Path and Angle also appear.

Detail is the one worth understanding: it splits marks more finely without any visual change, which is how you get the underlying rows separated when a calculation needs them.