堑 is a bound form (b.f.) — it never appears alone as a word. It means 'trench' or 'moat' and is used in literary or formal compounds like 天堑 (natural barrier, lit. 'heavenly moat'), 堑壕 (trench, as in warfare), and the set phrase 吃一堑，长一智 ('a fall in a pit, a gain in your wit' — learn from setbacks). You'll mostly encounter it in elevated writing or idioms.