This is the type of stringing of the piano (usually applied to upright pianos - almost all grand pianos are overstrung). Overstrung pianos have their bass strings fitted diagonally from the top left of the piano (uprights) to the bottom right "over" the treble strings which go from top right to bottom left. All modern pianos are overstrung apart from a very few ultra small short compass examples.
The terms overdamped and underdamped apply to upright pianos only. Overdamping is where the dampers are above the hammers, near the top of the strings. This method of damping is not as effective as underdamping and the notes on an overdamped piano often tend to "ring on", this is where the note continues sounding even when the key has been released.
Underdamping is where the dampers are below the level of the hammers. The dampers are near the middle of the strings and so the damping is quite effective. This arrangement usually gives a nice clean note cut off once the key is released.
https://www.pianoshop.co.uk/info/faqs.phpI had to look it up myself
