源 is a bound morpheme (b.f.) meaning 'source' or 'origin'. It cannot stand alone as an independent word; instead, it appears as a root in compound words such as 来源 (láiyuán, 'source'), 资源 (zīyuán, 'resources'), and 起源 (qǐyuán, 'origin'). You'll encounter it in both concrete contexts (e.g., the source of a river) and abstract ones (e.g., the cause of a problem).