Learn Russian: Это парк.

Look at the last letter! парк ends in a consonant (к). In Russian, nouns that end in a hard consonant in their dictionary form (the nominative case) are almost always masculine.

If парк is masculine, why does the sentence use the neuter word это instead of the masculine этот?

When you are pointing something out and saying "This is a...", you always use the fixed word это, no matter what gender the noun is. You would only use the masculine этот if you were attaching it directly to the noun to say "this park", like in the sentence Этот парк большой (This park is big).

Where are the words for "is" and "a" in this sentence?

Russian doesn't have articles like "a" or "the". Additionally, the verb "to be" (is, am, are) is dropped in the present tense. Because of this, "This is a park" is simply expressed as Это парк.