From the release notes:
A new “effects” log has been added to chat to help new players recognize when they have item or spell effects that don’t stack. This option can be turned off in the “incoming chat” options list.
Thank you! Although I'm not exactly 'new' I still get very confused in this game about what stacks and what doesn't stack, especially when many items have multiple effects.
I noticed this when equipping my Veteran artificer gear, fresh off the boat at Level 4, which contained an amazing number of non-stacking effects.