shbsssn Extended Support Releases (ESR) are typically for 1 year past a devices end-of-life (EOL); a stopgap for users to transition to the far more secure current generation devices.
This may include kernel updates (not provided by the vendor), partial security updates - only half the Android security updates are provided by AOSP (the patch level is not up-to-date for end-of-life devices). i.e., there will be no security updates for the firmware, no ongoing support for any of the platform drivers, driver libraries. no more support for the code from the vendor. Full security updates are not possible.
We explain it in the second part of this section as well: https://grapheneos.org/faq#supported-devices