Totem giants are a particular race who are especially good at manipulating incarnum, the magical force that powers everything. Avid worshippers (and hunters) of magical beasts, totem giants tattoo themselves with symbols of their favourites. These same tattoos are clues as to what kind of powers they are likely to manifest in combat. Totem giants can be found anywhere in the world, since they often use their incarnum powers to be able to survive in environments that would otherwise be deadly.
Okay, so D&D has soooo many different kinds of magic in it. Most people just stick to the ol' arcane-divine group because magic is already complicated enough and there's twelve bajillion spells without adding an oh but my magic works THIS way and bluhhhhhhh. I haven't that much into how incarnum works, but it honestly sounds kinda cool? The amount of spells you have to choose from (and are able to cast) is limited compared to the rest of the stuff, but it's kinda nice that way.
Basically you shape incarnum (this blue Force stuff) into this quasi-physical stuff called soulmelds. They're kinda like spells but also kinda like magic items you wear. But you can still use non-spell magic items you're wearing. Unless you super-bind them to you then you can't use magic items, but the incarnum spell is extra strong and give your a lot of cool stuff. And you can have a bunch of them active at the same time. It's complicated.
Okay. For example. The totem giant in the book has the Frost Helm spell. If they just manifest the Frost Helm normally, it basically allows them to live in super-cold environments and get resistance to cold. If you were wearing some magic item on your head (ex: circlet that grants you telepathy) you'd still be able to use the telepathy. But if you super-bind the Frost Helm spell you can get a buncha cool abilities (a cold breath weapon or a stunning sonic shriek), but the power of the spell overrides the power of whatever magic item you have at the time.
Like I said, it's complicated.
But I kinda wanna play this system and use this giant chick? I probably wouldn't be able to use her since she's a giant and that's not a playable race. I guess I could use a half-orc or a goliath but it's not the saaaaame. I wanna be huuuuuge.