Uname -r on arch

$uname -r
Linux kernel ver. 5.9
*5-kernel version
*9-major version
*1-minor version
what is arch1-1 means ?

The -1 at the end is probably the pkgrel. That means that it is the first package for that release of the kernel. All packages have something similar at the end.

The arch1 is part of the version string the package maintainer is using for the kernel.

You can see it if you review the PKGBUILD for the kernel

It appears to have been added to the pkgver with this commit for kernel 5.4 but the commit does not reference the reason.