Learn Russian: Восемьсот метров — это далеко.

The word for 800 is made of two parts: восемь (eight) and сот. The second part is the genitive plural form of сто (hundred). For numbers 500 through 900, Russian literally says 'five of hundreds', 'eight of hundreds', and so on.

Why does the word метр change to метров here?

Метров is in the genitive plural case. In Russian, numbers 5 and above (which includes hundreds like 800) require the noun that follows to be in the genitive plural. For a masculine noun ending in a consonant like метр, you add -ов.

Why do we need the dash and the word это in this sentence?

Because Russian drops the verb 'to be' (is/are) in the present tense, a dash is often used to link the two parts of a sentence. Adding это (literally 'this is') acts like a bridge, making it clear that the distance is the thing being described as 'far'.

Why does the sentence use the adverb далеко (far) instead of an adjective?

When describing a general fact, a quantity (like 800 meters), or a concept rather than a specific physical object, Russian uses the adverb form ending in -о. If you were describing a specific road, you would use a matching adjective, but for a general distance, the adverb далеко is needed.