Amazon’s cloud based Simple Storage Service (hence the S3). It is, in ELI5 terms, a file storage service like Microsoft’s OneDrive or Apple’s iCloud. A bit like a harddrive reached over the internet. You transfer files to and from it.
That’s grossly oversimplifying of course. S3 has some important technical differences to a straight up “cloud drive”. And it comes with a mind boggling array of options so it can be customised to be the storage of all sorts of very large real world applications and websites.
While these are both certainly true it has also been annoying when I’ve added a regular contact but it doesn’t show up in WhatsApp straight away