It's like S3, except better because, by focusing on being a write-only data store, they can manage much more throughput and efficiency, plus your data is far more secure at rest than it is in S3.
My company is a Fivetran client, and they named that company after a (bad) joke, but it's worth a fortune.
[1] https://news.ycombinator.com/item?id=42434450