So exactly what does this message mean and exactly what do I need to do.
Previously I've just repeated the command and the nag message went
away but this time it stays.

user@system: sudo apt-get update
Get:1 http://deb.devuan.org/merged chimaera InRelease [33.9 kB]
Get:2 http://deb.devuan.org/merged chimaera-security InRelease [21.5kB]
Get:3 http://deb.devuan.org/merged chimaera-updates InRelease [26.5 kB]
Get:4 http://deb.devuan.org/merged chimaera-backports InRelease [19.4 kB]
Get:5 http://deb.devuan.org/merged chimaera/main amd64 Packages [8,311 kB]
Get:6 http://deb.devuan.org/merged chimaera/main i386 Packages [8,248 kB]
Get:7 http://deb.devuan.org/merged chimaera/main Translation-en [6,482 kB]
Get:8 http://deb.devuan.org/merged chimaera-security/main amd64 Packages [111 kB]
Get:9 http://deb.devuan.org/merged chimaera-security/main i386 Packages [111 kB]
Get:10 http://deb.devuan.org/merged chimaera-updates/main amd64
Packages [2,608 B]
Get:11 http://deb.devuan.org/merged chimaera-updates/main i386 Packages [2,612 B]

Reading package lists... Done
E: Repository 'http://deb.devuan.org/merged chimaera-backports InRelease' changed
its default priority for apt_preferences(5) from 500 to 100.
N: This must be accepted explicitly before updates for this repository can be
applied. See apt-secure(8) manpage for details

Except man apt-secure doesn't give me anything I understand.

TIA.