What is a ruler foot? I have been asked that question so let me attempt to explain.
On a sewing machine, a foot is always used to hold down the fabric while it is stitched.
This is my go-to foot to use on my machine. It fastens onto the machine and the needle goes up and down in that opening.
There are feed dogs under the foot that pull the fabric through the machine while it is stitching.
When a person does free-motion quilting, the feed dogs are lowered because the person guiding the fabric doesn't want to fight with the feed dogs feeding front to back. Fabric can be moved in any direction when free-motion quilting. So the feed dogs are lowered and a free-motion quilting foot is used. Here is the foot I use.
The foot is open so I can see the needle easily. This foot bounces on the fabric so it does not continually hold the fabric like a traditional foot. This bouncing allows me to move the fabric as I am stitching.
A ruler foot is another free-motion quilting foot. It acts like a regular free-motion foot but the bottom is taller. The foot still bounces on the fabric but it is tall/thick enough that it will not inadvertently bounce onto the ruler and shatter a needle.
In the following picture, the regular free-motion foot is on the left and the ruler foot is on the right.
Why use free-motion rulers? I can stitch straight lines easier as well as circles and other shapes. My shapes are more uniform.
Hope that helps!