This is the putty or rubber gasket holding the actual glass pane into the frame. For a (non-opening), this is the primary seal. Symptoms: Visible gaps between the glass and the frame, or condensation inside a double-pane unit.
: Open the window, place a piece of paper between the frame and the sash, and close it. If you can pull the paper out easily with no resistance, the seal is bad. For Fixed Frames how to reseal a window fixed