A flat adverb is an adverb that looks exactly like its adjective form. For example, "fast" can be both an adjective (describing a noun) and an adverb (describing a verb or action). Instead of adding "-ly" to the end of the word to create an adverb, flat adverbs are used without any changes.
Flat adverbs often end up being less confusing than their -ly counterparts because they look exactly like the adjectives we use every day. For example, we might say, "She ran fast" instead of "She ran quickly." They both mean the same thing, but the first sentence uses a flat adverb instead of a regular adverb.
Some other examples of flat adverbs include, "hard," "deep," "close," and "quick." Just remember that a flat adverb is a word that can be used as both an adjective and an adverb without any changes to its spelling or form.