Each land is a Forest in addition to its other land types.
Yavimaya is one of the most quietly powerful lands in green Commander, and it's an auto-include in a huge swath of decks for two distinct reasons.
First, mana fixing: turning every land (yours and your opponents') into a Forest means all your lands tap for green. In multicolor green decks, this smooths your curve and helps cast green spells off-color lands. Pairs absurdly with anything that costs .
Second, and more importantly, synergy payoffs:
- Lands matter / Forest synergies: powers up cards like Nissa, Who Shakes the World, Mana Reflection, Nykthos, and dorks that ramp off Forests.
- Reki, Multani, Argothian Elder, and Forest-fetching effects all benefit.
The downside: making opponents' lands Forests can backfire—it enables their green effects and forestwalk. It also doesn't tap for any color you aren't already in.
Skip it in mono-color non-green decks or decks with no Forest/green payoffs—it's just a tapland-quality Forest then. Otherwise, in green, it's a near-free upgrade.
Land cards not on the battlefield aren't Forests while Yavimaya is on the battlefield.
Yavimaya's ability causes each land on the battlefield to have the land type Forest. Any land that's a Forest has the ability ": Add ." Nothing else changes about those lands, including their names, other subtypes, and whether they're legendary, basic, or snow.
: Add 
