Learn Russian: Это чистейший эфирный экстракт.

It uses the morphological superlative suffix -ейш-. You take the stem of the adjective чистый (clean or pure), which is чист-, add -ейш-, and then the standard masculine adjective ending -ий to match the masculine noun экстракт.

What is the difference between saying чистейший and самый чистый in this context?

Самый чистый (the analytical superlative) is typically used for direct comparison, meaning the purest out of a specific group. The morphological form чистейший acts here as an absolute superlative. It means extremely pure or of the highest conceivable purity, emphasizing the intense quality of the extract rather than strictly comparing it to other samples.

Why does эфирный translate to 'essential' here instead of its literal meaning?

The adjective эфирный literally means ethereal or relates to ether. However, in the context of plant extracts and oils, it is the standard Russian term for essential. You will frequently see it in the very common phrase эфирное масло which means essential oil.

Why is the pronoun это neuter when the noun it introduces, экстракт, is masculine?

When это is used as a dummy subject meaning 'this is' or 'that is' to introduce or identify a noun, it always remains in the neuter singular form. It does not change to match the gender or number of the noun that follows.