From an optimization point of view, it means that if a rectangular area's edges contains only values reached the iteration limit without growing larger than the escape value, the entire area belongs to the Mandelbrot set.
Simply-connected sets and compact sets are both sometimes informally described as "sets with no holes," but the definition of "holes" is different in both cases.
A compact set contains all of its limiting points (i.e. limits of convergent sequences of points within the set). Therefore it may not have any point-shaped holes. But it may still have a hole shaped like, e.g. an open disc.
To rule out such large holes, you want a simply-connected set, which is a connected set in which any closed path can be continuously shrunk to a point.
For those that want to fly around in the mandelbrot set, even a 2015 desktop can easily keep up, I suggest Xaos which I believe includes all the common mandelbrot optimizations at https://xaos-project.github.io/
There's even a web version at the above URL. It's in most linux repos I tried apt or yum. I couldn't find any docs for what optimizations Xaos uses, but it seems plenty fast for real time zooming, even on old hardware.
Or you can set up a grid and calculate the chunks.
When I was learning a new language in my collage days, my helloworld used to be implementing a Mandelbrot/Julia set on the given language. A few months ago I decided to look into Rust and started to work on my helloworld in Rust, using RayLib :-)
Here's an example output from my version: https://ibb.co/p4bFF7S
There was a Mandelbrot/Julia set renderer on the Atari ST that did the exact same thing. Instead of waiting rather long for a pixel-perfect image you could see output immediately and it got more refined with each recursive stage. On a box with a 68000 CPU @ ~8 MHz and no FPU it made for a really nice effect.