Spacers are exceptionally useful. They have one very basic, but very important purpose – they add vertical space between your blocks. Let's say you've got two blocks that are hard up against each other. You'd want to insert a spacer between these two blocks. To do so, click 'add to', find the spacer, click 'add' and then move the spacer where you'd like it.
When you go to add a spacer, it defaults to 10 pixels, which is perfect for most sites. If you'd like to your spacer to be larger or smaller than that, you can easily edit this space by entering in a higher or lower number. Just keep editing the number until your space looks just right.