一席话 is a fixed phrase that literally means 'a seat of words' — it refers to a complete stretch of speech or a meaningful conversation, often in contexts where someone's words have a significant impact. The classifier 席 is specially used for conversations, speeches, or banquets. You'll hear it in the classic saying '听君一席话，胜读十年书' (Tīng jūn yī xí huà, shèng dú shí nián shū — 'Listening to your words is better than reading ten years of books').