な-Adjective だ

Na-adjectives describe a property of a noun, and when you want to state that property as a simple fact, you present the na-adjective in its predicative form with だ in plain speech or です in polite speech. The だ/です attaches to the na-adjective stem (which ends with な in the dictionary form) to turn it into a predicate describing the subject of the sentence. This is the most basic, neutral way to make a statement about what something is like.