This lintel is old, and the sliding door doesn't slide smoothly.
敷居 is the grooved threshold at the bottom of a sliding door frame, while 鴨居 is the corresponding lintel at the top.
Standard kanji spelling for this architectural term.
The origin of the word is uncertain. The kanji 鴨 (wild duck) and 居 (reside) are ateji, chosen for their sounds rather than meaning.