It means 6.4.4.
The 0 can be ignored, so you can think of this as "linux-image-6.4" then if you want the actual revision you look after the arch and see "6.4.4".
This is explained in the Debian kernel handbook:
https://www.debian.org/doc/manuals/debian-kernel-handbook/ch-versions.html
For compatibility, the official kernel packages currently add '.0' to the upstream version.