Learn Spanish (Latin America): El desierto es más seco que el bosque.

In English, we often add "-er" to the end of short words to compare them (dry becomes drier). Spanish doesn't use endings for this. Instead, you just put más (more) before the adjective. So, "drier" is simply expressed as "more dry": más seco.

Does the word que mean "than" here?

Yes! You might remember que meaning "what" or "that" from earlier lessons. But when making a comparison, the formula is más + [adjective] + que. In this context, que translates to "than."

There are two nouns here (el desierto and el bosque). Which one does the adjective seco agree with?

The adjective always matches the subject of the sentence, which is the thing you are primarily describing. Since el desierto is masculine and singular, we use seco. If the sentence started with a feminine noun like la región (the region), the adjective would change to match it: La región es más seca...

Why do we use es instead of está to talk about being dry?