When saving a PNG image, what does "interlaced" mean?

Interlacing is done for web graphics. It has to do with how the images load on a web page. Noninterlaced images load in rows while interlaced images load with random pixels in each row, so they begin by looking blurry (low res) and then sharpen as they load. It's done so when you load a webpage, you can see the contents at a glance, but as the page loads, it becomes sharper.

Interlacing increases file size, so unless you are using the .png for the web, choose "no interlacing" on .pngs.