槛 (jiàn) is a bound form (不成词语素) that appears in compound words meaning 'threshold' or 'door sill'. It cannot stand alone as a word — you'll always see it paired with another character, most commonly in 门槛 (ménkǎn, 'threshold', but note the pronunciation shifts to kǎn in that compound). In formal or literary contexts, 槛 (jiàn) also appears in words like 槛车 (jiànchē, 'prison van') and 兽槛 (shòujiàn, 'animal cage'). Be careful: the same character has a colloquial reading kǎn used in everyday speech for 'threshold'.