Framelines are visual guide lines seen in a viewfinder that indicate what will be captured in the image. Most cameras nowadays just use the entire viewfinder space to show what will be captured, but this was an essential feature for older cameras, especially those like rangefinders where it isn't possible to view through the lens.

Here's an image from a rangefinder I own, the Fuji GW690.

Note that we are definitely not looking through the lens here as the top-left corner of the lens & hood are visible in this shot. Because rangefinders don't have a mirror available in the line of sight to split/alternate the view between the viewfinder and film, a separate view is used with frame lines to show the capture area. Also note that rangefinders may come with alternate viewfinders mounted on the top shoe or on a spot on top of the camera to support views with other lenses whose field of view may not match the defaults on the camera. Some cameras also had multiple frameline boxes, each to support a specific lens known to be available for the camera.