Adjectives in Russian have a long form and a short one. A long one is used before nouns and as a part of a predicate. A short "predicative" form can ONLY be used as a predicate (with a "to be" verb). Usually it looks the same as an adverb.
To be more exact, that typically refers to adjectives that mean qualities that exist on a sliding scale (good, short, soft etc.) rather than yes/no characteristics (wooden, American).
Their usage depends on the meaning and style and isn't all that easy. It is especially frustrating for a learner when a short and a long form are interchangeable but modify the overall tone.
Now, from a practical point of view, a number of adjectives are used in a short form very often (or always) and/or have a distinct meaning then. It is safe to say that the majority of adjectives in the language aren't much used in short form, so better learn those that are.
That's what we are going to do. And you get adverbs for free.