[YeGoblynQueenne]

They are dual lands with basic land types without any abilities (other than the mana abilities they inherit from their basic land types) which also means without any drawbacks. They're just "Swamp Forest", "Mountain Forest", "Island Swamp", etc. See here:

https://mtg.fandom.com/wiki/Dual_land

And they are literally what their type lines say. So you basically get two lands with a basic type for the price of one (i.e. one card, or one land drop; card and tempo advantage, 2-in-1). Their only limitation is that they are not basic lands so they are subject to the restriction of number of copies per deck (4, in most formats).

Every dual land created subsequently has some kind of drawback or limitation (other than the number of copies restriction): "Enters the battlefield tapped", paying some amount of life or taking damage, sacrificing a permanent, discarding a card (I think), printed as bouble-faced card, and so on.