What exactly does the enable-background attribute do?

As noted by Robert Longson in comments, attribute enable-background is deprecated at least since 2014.

SVG 1.1 introduced the enable-background property. The property defined the back drop under the filter region at the time that the <filter> element was invoked. The concept defined by this property was identified to be incompatible with the model of stacking context in CSS at the time writing this specification. UAs can choose to implement the enable-background property as defined in SVG 1.1 but will not be compatible to this specification or to CSS Compositing and Blending [W3C Working Draft].

It's supposed to make the background image available to child elements of the element it's specified on for things like filter effects that blend content with the background. There may be other uses, but that's the one I know.

If you don't have it set, then technically the element can't use backgrounds created by ancestors.

The only major browser that supported it (ever) was IE10/11, so it doesn't get used very much. (It's also there as boilerplate in every Illustrator SVG export - for no good reason.)

(It's also been deprecated by all the major browsers since 2014)