简 is a bound form (morpheme) that cannot stand alone as a word but appears in many common compounds. Its core meaning is 'simple, brief' — you'll find it in words like 简单 (simple), 简化 (simplify), and 简历 (résumé). Historically, 简 referred to bamboo slips used for writing before paper, which carries over into 竹简 (bamboo slips) and 书简 (letter). A less common, more formal meaning is 'to select' in compounds like 简拔 (to select and promote). Context tells you which sense is at work when you see 简 in a compound.