An adjective describes a noun. This can be its appearance (like a color), its size, its shape or anything else that paints a more vivid picture of the noun. Examples include "blue," "large," "fat," "easy," "hard" and "viscous." The adjective doesn't necessarily have to describe a physical characteristic. If you write, "The movie was sad," you are not describing anything that can be immediately seen. Rather, you are describing the emotion that watching the movie evokes.
Adjectives do not have to be one word. They can also be groups of words preceding a noun, such as "big red bike." While "big" and "red" are two words, they are actually one adjective in this case, because they are describing the same noun.
Adjectives can be used with the verb "to be" and all its forms. This means you can write, "The bike is large" rather than "There is a large bike." This makes it possible to construct sentences in a variety of ways.
In English, an adjective almost always comes before a noun. "The big car" is correct sentence structure; "the car big" is not. The adjective can only be placed after the noun when it is connected with the verb "to be": "the bike was big" or "the bike is big" are both acceptable as well.