Inline CSS lets you apply styles directly to a single HTML element by using the style attribute. The syntax places the style attribute inside the opening tag of the element you want to style. Inside the attribute, you write one or more CSS property-value pairs, separated by semicolons. For example, style="color: red; font-size: 20px;" will set the text color and size for that element only.

Inline styles have the highest specificity compared to external or internal CSS, meaning they override any conflicting styles set elsewhere. However, using inline styles can make your code harder to maintain, so it is best reserved for quick changes or unique cases.