Short answer
It's the address your files are served from, such as a custom domain or CDN in front of your bucket (for example files.example.com). Aktar joins it with the object path to build the link it copies.
Aktar keeps the upload endpoint and the public address separate on purpose. Files are uploaded through the provider's S3 API endpoint, but people open your links through a public domain, and the two are almost never the same host.
Example: a Public Base URL of https://files.example.com and an object key of 2026/09/7f3c2a91.png give the link https://files.example.com/2026/09/7f3c2a91.png. If you leave out https://, Aktar adds it for you.
For the link to open for other people, the bucket (or the domain in front of it) must allow public reads. The Test Connection button checks this for you.