Learn Spanish (Latin America): Es importante que leas la etiqueta.

Because the sentence uses the impersonal expression Es importante que... (It is important that...). In Spanish, impersonal expressions that state a judgment, opinion, or suggestion require the verb in the following clause to be in the subjunctive mood. Leas is the present subjunctive form of leer for tú (you).

Could I just say Es importante leer la etiqueta instead?

Yes, but it changes the meaning slightly. Using the infinitive (leer) without que makes it a general statement: "It's important to read the label" (for anyone). By using que leas, you are making it personal and directing the statement specifically at the person you are talking to: "It is important that you read the label."

Why isn't the word tú included before leas?

Because the verb ending already makes it clear. The -as ending on the subjunctive verb leas tells us the subject is tú (informal you). You could say Es importante que tú leas la etiqueta for extra emphasis, but native speakers typically drop the pronoun since it isn't strictly necessary.

Does etiqueta mean "etiquette" or "label"?

In this context, it means "label" (like a tag on clothing, a price tag, or a nutritional label on food). While it can sometimes translate to "etiquette" (good manners) depending on the context, "label" or "tag" is its most common everyday translation.