Learn Romanian: Această brânză este foarte bună.

Brânză is a feminine word. We use acest for masculine or neuter nouns, and această for feminine nouns. You might also see aceasta, but that form is used when it stands alone as a pronoun meaning this one, or when it comes after the noun, as in brânza aceasta.

Why does the word for good end in an ă (bună)?

Adjectives in Romanian change to match the gender of the noun they describe. Since brânză is a feminine noun, the basic masculine adjective bun changes to its feminine form, bună.

If we are talking about a specific cheese, why don't we use the form for the cheese (brânza)?

When you put a pointing word like această before the noun, the noun stays in its basic dictionary form. The word această already points out exactly which cheese we mean, so we don't add the extra ending for the.