Certificates expire, and you have to make a new one. That has always been a pain point. See Proper way to renew distribution certificate for iOS for what we used to have to do; there was an old way where you had to start certificate creation from your keychain, and a newer way where you could just make the certificate at the developer portal and it would be installed on your computer for you. So things have gotten better over time, but expiration still would require action on your part. Plus, if you wanted to renew your certificate before it expired all by itself, you had to revoke it first.
By default, Apple now removes this pain point entirely by automatically revoking and replacing (“rotating”) the certificate for you, so that, provided you do some developing at fairly frequent intervals, it all just happens behind the scenes, with no friction.
If you want to renew the certficate now (perhaps because you won’t be doing any developing for the next four months but you would like a current certificate to be in place four months from now), tap the “Manually Rotate Certificate” button, and the whole revoke-and-replace cycle will be performed for you (exactly as would have happened automatically anyway, provided you did some fairly frequent developing).
Renewal of a certificate has no effect whatever on existing apps. You do have to download the new certificate, obviously, because it must live in your keychain; but nowadays you do not have to download it manually, as this is done for you by Xcode.
