困难 (kùnnan) means 'difficult' or 'difficulty' — it works as both an adjective (a hard task) and a noun (obstacles one faces). Compared with 难 (nán), the common HSK3 word for 'hard to do', 困难 emphasizes the sense of a challenging situation or obstacle. Other synonyms like 艰苦 (jiānkǔ) and 艰难 (jiānnán) highlight tough conditions or an arduous process, but 困难 is the most general all-purpose word.