Adds a rock based species that eats minerals instead of food and has a few unique traits available. The rock species specific origin makes your rock people expand via meteorites.
Honestly not worth the full price, get on discount.

Do note that the devs in progress of rebalancing and adding more things to each of the race DLCs, possibly making these more worthwhile.

I'm glad to see this DLC available for Stellaris - the lithoids in Master of Orion 2 were weird and interesting to me when I was younger. Living rocks? That eat rocks? Crazy. But beyond adding living rocks to Stellaris, there's not much that this DLC does that you need. If you want rocks, get some rocks. Otherwise, skip the rocks.

Like most species packs it doesn't add a lot, but a must if you want to play as Lithoids. If you don't you're not missing anything.