Learn Spanish (Latin America): Ella se viste con ropa bonita.

Viste comes from the reflexive verb vestirse, which literally means "to dress oneself." The se is the reflexive pronoun for "herself," so ella se viste literally means "she dresses herself." Without the se, it would mean she is dressing someone else, like a child.

The dictionary says "to dress oneself" is vestirse. Why is it spelled viste here instead of "veste"?

Vestirse is a stem-changing verb. That means the vowel in the middle of the word changes when you conjugate it. For this verb, the e changes to an i in most forms, so it becomes viste for "she dresses."

Why does the Spanish sentence use con (with) instead of a word for "in"?

In English we say we dress "in" clothes, but in Spanish, the standard phrase is vestirse con (literally "to dress oneself with"). Prepositions often don't translate word-for-word between languages, so it is best to memorize them together with the verb.

Why is it ropa bonita instead of plural ropas bonitas? Doesn't "clothes" mean more than one thing?

In Spanish, ropa is almost always treated as a singular noun, much like the word "clothing" in English. Because ropa is singular and feminine, the adjective describing it must also be singular and feminine (bonita).