The basic blocks of CSS are properties and values. Properties are a human-readable identifiers that describe what is being styled. Values indicate how to style that property. When these are paired together they create a CSS declaration

/* Property: color, value: red - together they form a CSS declaration */
color: red;

CSS declarations are found within CSS declaration blocks. When CSS declaration blocks are paired with selectors (or a list of selectors) they produce CSS rulesets (or CSS rules).

/* p is a selector which together with the CSS declaration block creates a CSS rule */
p {
  /* Multiple declarations form a CSS declaration block */
  background-color: black;
  color: white;
}

[[MDN]]. How CSS is structured. Link