Learn Portuguese (Portugal): A erva do parque está húmida.

You use the verb estar (here conjugated as está) because being damp is a temporary condition. If you used ser (like é), it would imply that being damp is a permanent, unchanging characteristic of that grass.

The English translation says "in the park", so why does the Portuguese sentence use do instead of no?

In English, we say the grass is "in" the park, but Portuguese prefers to link the two nouns using possession or origin, meaning "the grass of the park" (de + o = do). Saying no parque (in the park) wouldn't be completely wrong, but do parque sounds much more natural.

Does erva always mean grass, or can it mean something else?

Erva is a versatile word! It can mean wild grass, weeds, or even herbs (as in ervas aromáticas). If you are talking specifically about a planted, manicured lawn, people in Portugal more commonly use the word relva.

How do you pronounce the "h" in húmida?

You don't! In Portuguese, an "h" at the very beginning of a word is always completely silent. You just pronounce the vowel that follows it. Also, notice that húmida ends in "a" so it agrees with the feminine noun a erva.