Prohibido Consumir Bebidas Alcohólicas
This sign in Spanish is from a local park:
Prohibido comes from the verb Prohibir which means to Prohibit or not allow or not permit something.
After Prohibido we normally have the verb in the infinitive form (ending in AR, ER or IR)
In this case it is the verb Consumir which means to consume, to eat or to drink depending on the context.
Since Bebidas (drinks) is a noun in Plural Feminine form, the adjective that describes it should also be in the same form, in this case alcohólicas (alcoholic).
Notice the difference between:
Bebidas Alcohólicas (Plural Feminine)
Tragos Alcohólicos (Plural Masculine)
Trago is a more casual way of saying a drink in Spanish so is not appropriate for an official sign.
A quick vocabulary summary:
Prohibir = To prohibit, not allow, not permit
Consumir = To consume, to eat, to drink
Bebidas = Drinks
Beber = To Drink
Alcohólico = Alcoholic
Bebidas Alcohólicas = Alcoholic drinks