The French prefix a- is added to adjectives and nouns to create their antonyms; it's sometimes called the "alpha privative" (a privatif). The most common English equivalents are a- and non-.
French adjectives may be found before or after the nouns they modify, depending on various factors. Generally speaking, descriptive adjectives follow nouns, while limiting adjectives precede nouns.
An adjective is something like "a word that describes a noun." If asked to name some adjectives, you might come up with words like small, pretty, and blue - in other words, descriptive adjectives. But did you know that there are many other types of adjectives as well?
One of the eight parts of speech, adjectives are a type of modifier; that is, they modify or describe nouns in a certain way, letting you know the size, shape, weight, color, nationality, or any of a myriad other possible qualities of nouns.
By definition, adjectives modify nouns. But more than 30 French adjectives can sometimes modify verbs instead, thus taking on the role and characteristics of adverbs, including the fact that they are invariable.
In both French and English, many adjectives can be used as nouns as a sort of shorthand to reference what you'd otherwise need an adjective + noun to refer to.