Learn Finnish: Hänen ruokavalionsa on lihaton, koska hän on kasvissyöjä. - His diet is meatless, because he is a vegetarian.

It is made by taking the noun liha (meat) and adding the suffix -ton, which means "without" or "-less". This turns the noun into the adjective lihaton (meatless).

Why use -ton here, but sometimes -tön on other words?

It depends on vowel harmony! Words with back vowels (a, o, u) like liha take -ton. If the base word has front vowels (y, ä, ö), like työ (work), you must use -tön instead, making työtön (unemployed).

Why does ruokavalio (diet) have the ending -nsa?

In standard Finnish, when you say "his" or "her", you use the genitive pronoun hänen AND add the third-person possessive suffix -nsa (or -nsä) to the noun. So hänen ruokavalionsa is a mandatory double-marking that essentially means "his diet-his".

kasvissyöjä (vegetarian) looks like a very long word. What are its parts?

It is a compound of two words: kasvis (vegetable) and syöjä (eater). The word syöjä comes from the verb syödä (to eat). So a vegetarian in Finnish is literally a "vegetable-eater".