New Content would be new things, from biomes to objects to mechanics to enemies.
As opposed to things that are simply minor tweaks to existing content.

In some cases the difference can be debatable, as its hard to definite the line between changes to certain things and what's basically an entire new rework of something.
H&H contains new building parts, foods, enemies, status effects, weapons, shields and new mechanics, large overhauls of existing mechanics, and a ton of small but impactful QoL tweaks.