What an expiring link is
A presigned URL is a normal web link with your permission baked in. Your client signs it locally with your credentials, and the signature says: whoever holds this link may download this one file, until this time. The recipient clicks and the download starts. No account, no app, no login screen.
Nothing about the bucket changes. It stays private; the link is the only key, and it stops working the moment it expires.
The rules worth knowing
The lifetime is yours to pick, from minutes up to seven days on Amazon S3 and most compatible services. Anyone holding the link can use it, so treat it like the file itself: send it over a private channel, not a public chat. And the link is signed by your credentials, so if those keys are deactivated, the link dies with them.
Making one without the console
The AWS console can create presigned links, but it means logging in, digging to the file, and repeating the dance for every link. In Varv it is a right click: pick the file, set how long the link lives, copy, send. Share links work on any connected S3 service, and for versioned buckets you can link a specific older version of a file too.
When an expiring link is the wrong tool
A link that dies after a week is for handing a file to a person. If a website needs to serve files to everyone forever, that is a public bucket behind a CDN, a different setup with its own tradeoffs. If a program needs ongoing access, give it real credentials scoped to what it needs instead of a link you keep refreshing.