What does it mean that context-dependent memory?

You remember things better in the context where you learned them.

Example: You study a software architecture concept at your desk with coffee, lo-fi music, and two monitors. Three weeks later you're in a conference room whiteboarding with six people and you can't recall the pattern — only a vague sense that you knew it once. Back at your desk that evening, the concept surfaces immediately. The knowledge didn't leave. The retrieval context changed.

Try this: Pick a concept you recently studied. Close your eyes and mentally reconstruct — in detail — the physical environment where you learned it: the room, the lighting, the sounds, what you were drinking, what was on your screen. Hold that scene for 30 seconds. Then try to recall the concept. Compare the richness of that recall to what you could retrieve before the mental reinstatement. The difference is the context-dependent retrieval effect operating in real time.