Adjectives describe or qualify a noun. In Spanish, they have to match gender and number (this does not happen in English: we can say the red guitar, for singular, or the red guitars, for plural, and red remains the same).
Generally, adjectives in Spanish are placed after the noun:
El piano rojo, los pianos rojos, la guitarra roja, las guitarras rojas (for specific pianos or guitars),
un piano rojo, unos pianos rojos, una guitarra roja, unas guitarras rojas (for non-specific pianos or guitars).