就是 is a versatile two-character expression that functions primarily as a conjunction or adverb, emphasizing precision, confirmation, or clarification. Literally, 就 means 'just' or 'exactly', and 是 means 'is' or 'to be'; together, they form a cohesive unit meaning 'exactly', 'precisely', or 'that is to say'. It’s commonly used to reinforce identity, definition, or agreement — for example, when confirming someone’s identity ('That person? That is exactly the teacher I mentioned') or clarifying a point ('It’s not complicated — it’s simply this').

The phrase often appears in spoken Mandarin to add emphasis or resolve ambiguity, especially after a noun, pronoun, or clause. It frequently follows demonstratives (like 这、那) or questions (like 谁、什么), and can introduce explanations or corrections. Unlike standalone 是, 就是 adds nuance of certainty and immediacy — not just stating a fact, but asserting it with confidence or finality. It’s neutral in register and widely used across contexts from casual conversation to formal explanation.