It's an adjective that is used for both masculine and feminine singular nouns. It can also be used in its plural form, meilleurs.
Here are some examples:
* C'est un meilleur choix. (This is a better choice.)
* Elle est une meilleure cuisinière que moi. (She is a better cook than me.)
* Ce sont les meilleurs restaurants de la ville. (These are the best restaurants in the city.)