Basically it means you want your colors in the image to run from white to black (usually). Are you familiar with a histogram? The idea is that you’d want your histogram curves to cover the entire range to get maximum contrast.
Generally you can do that by eye, but on a technical level a histogram is the way to verify what you’re seeing.
There have been a few threads about preparing images for engraving, let’s see…
Here’s one:
There are more, check out this search:
https://community.glowforge.com/search?q=Preparing%20engraving