Learn Portuguese (Portugal): O algodão é suave na pele.

In Portuguese, when you talk about general concepts, categories, or materials as a whole, you must use a definite article. Since algodão is a masculine word, we use O.

Why do we use é here instead of está?

We use é (from the verb ser) because being soft is a permanent, natural characteristic of cotton. The verb estar is used for temporary conditions, so you would only use está if something was temporarily soft.

algodão is a masculine noun, so why doesn't the adjective suave end in an 'o'?

Adjectives that end in the letter 'e' in Portuguese, like suave (soft), are gender-neutral. They keep the exact same form for both masculine and feminine nouns. For example, you would also say a seda é suave (silk is soft).

Where does the word na come from in this sentence?

na is a contraction of the preposition em (in/on) and the feminine definite article a (the). Even though it ends in 'e', pele (skin) is a feminine noun, so it takes the article a. Combined, em + a gives us na (on the).