The CSS filter property applies visual effects to elements — images, divs, text, anything. Adjust brightness to lighten or darken, contrast to punch up colours, blur for frosted glass effects, hue-rotate to shift all colours around the wheel, sepia and grayscale for vintage and monochrome looks, and drop-shadow for a more precise alternative to box-shadow. Stack multiple filters in one declaration and they apply in sequence. Use it on ::before or ::after pseudo-elements to filter backgrounds independently of foreground content.