Geometry interfaces is a CSS module that provides interfaces for working with 3D and 2D graphics — in particular, for working with points, rectangles, quadrilaterals and transformation matrices (for operations that translate/move, scale, rotate, skew/shear/slant, and flip graphics, as well as for multiplying/chaining and inverting/undoing those operations).
As a web developer, you don't always use the geometry interfaces directly, but instead use other features that rely on them behind the scenes: parts of CSS Transforms, the Canvas API, the WebXR Device API, and (more directly) VideoFrame.visibleRect, Element.getClientRects(), and Element.getBoundingClientRect().
DOMMatrixRepresents a transformation matrix, for operations that translate/move, scale, rotate, skew/shear/slant, and flip graphics, as well as for multiplying/chaining and inverting/undoing those operations.
DOMMatrixReadOnlyRead-only version of DOMMatrix.
DOMPointRepresents a 2D or 3D point in a coordinate system; it includes values for the coordinates in up to three dimensions, as well as an optional perspective value.
DOMPointReadOnlyRead-only version of DOMPoint.
DOMQuadRepresents a collection of four DOMPoint objects defining the corners of a quadrilateral.
DOMRectRepresents the size and position of a rectangle.
DOMRectReadOnlyRead-only version of DOMRect.
The Path2D.addPath() and CanvasPattern.setTransform() articles have examples that use some of the geometry interfaces.