Explain complexity, your approach, tradeoffs, and measurable result.
How to answer it
Choose a problem where the difficulty is explainable in two sentences and the solution had a measurable result. The interviewer is not grading the technology; they are grading whether you can identify what was actually hard and make a tradeoff.
Structure:
The problem and why the obvious approach failed. "Nightly attribution took 9 hours and was starting to miss the morning dashboard deadline. The query joined 2 billion events to sessions with a range condition, and the warehouse could not push the range join down."
The constraint. "No budget for a bigger warehouse; the result had to match the old one exactly, because finance reconciled against it."
What you tried and rejected, briefly. "Partitioning by day cut it to 6 hours; not enough. Pre-bucketing events into 15-minute windows let the range join become an equality join on the bucket plus a cheap filter."
The result, with a number and a verification. "Runtime went to 40 minutes. I diffed a month of output row by row against the old job before switching."
What it taught you that you still use.
Keep the jargon to what the story needs. A senior interviewer will ask a deeper question about any part; be ready to go one level down on the part you chose.
What they are checking: that you can name the hard part precisely, that you considered alternatives, and that you verified the result.
Common mistake: a tour of the stack ("we used Kafka, Spark, Airflow...") with no statement of what was difficult.