Window weights are lead weights that counterbalance the weight of a window sash - the movable part of the window - in older houses. They are common in double-hung windows, in which the two sashes move in independent tracks; the top one can be lowered and the bottom one raised, which allows circular airflow, hot air out the top and cooler air in through the bottom. Because older windows are made of heavy materials, raising a window can be difficult, and without counterbalance, the sash can come crashing down, potentially causing the panes to break. The solution is a pulley at the top of the window casement, with a cord attached to the sash, and inside the window frame itself, window weights that are sized to exactly or nearly equal the weight of the window sash. There are two window weights per sash, one on each side of the window.
The 'sash cord' is the rope that connects the window sash to the window weights. Over time, these ropes can wear out and sometimes break, causing the window weights to drop. Since the weight cavities, also called window weight wells, in the window frame sometimes run the entire height of the house, window weights can potentially fall from the second floor to the ground floor.
Weighted windows are difficult to open if only one of the two window weights is in place, so if you have a broken sash cord and a dropped weight, you must replace both the cord and the weight. Unless your house was built with small access panels opening into the weight wells, you will have to get access to the weight wells by removing part of the window frame. You can purchase new window weights and sash cords at housing renovation specialty stores online.
If you are renovating an older house with double-hung windows whose weights have long ago dropped to the bottom of the weight well, it may be tempting to put off replacing the window weights until later, propping open the windows with a stick or other obstruction. However, non-counterbalanced windows can be very dangerous, particularly if you have small children. It is recommended that you do not open the windows at all until you have replaced the window weights.