A feeler gauge is a measuring tool used to check the gap between two items. It is often used when rebuilding an engine to make sure part fitting tolerances are acceptable.
Bladed Feeler Gauges
When most technicians think of a feeler gauge, this is the tool that typically comes to mind first. It is a set of "blades" of various thicknesses that can be used individually or stacked to fill a small gap, and when nothing larger will reasonably fit in the gap, you add all the measurements on the blades you used in order to determine the gap size.
Spark Plug Gap Tool
A spark plug gap tool is commonly a circular tool with a ramped edge or hooks of varying degrees of diameter, either of which are used to check the gap between a spark plug's ground strap and center electrode. The tool's measuring portion is placed into the spark plug gap, and if it fits, the gap is roughly equal to or larger than the selected measurement.
Brake Pad Feeler Gauge
A brake pad feeler guage allows for easy inspection of brake pad wear. They are typically color coded green, yellow, and red to allow a technician to easily show a customer how thin their brake pads have worn in a simple way without numbers or photos of complicated-looking parts.
Sometimes there are brake calipers whose designs don't allow these feelers to get in for an accurate measurement, but most calipers allow room to use this gauge to at least see when the brake pads are worn down to the red and yellow regions.