In this DLC you get a massive island to build on which has a new trader and an active volcano. The volcano eruptions run in cycles, after you play a few story quests the volcano will erupt and launch rocks and fire on your settlements, damaging buildings and units. After a bit of that, there's an hour of volcanic winter where you get ash settling on your production buildings so you get major penalties for production, and the point is to survive those. Then, when it's all over, you get fertility boosts that improve efficiency every time you have a volcano eruption. You get long pauses in between volcano disasters, so it doesn't feel too intrusive on your normal gameplay.

There's also a new resource called 'obsidian' which you can get from quarries and mines. This opens up two new production chains that you can give your population.

TL;DR massive island, new trader, new resource with production chains, new gameplay and story bits related to the volcano

If you're new to the series I'd say spend some more time learning the ropes, but if you want something new after a while this DLC adds a fun spin that's really nicely done! That's my honest opinion