What is crop factor?

Here is what the site says about that.

From Focal length converter: How it works: Multiply the focal length by the sensor's crop factor to get its full-frame equivalent. To go between two smaller formats, multiply by the first crop factor and divide by the second. Crop factors are measured on the sensor diagonal, so formats with a different shape (4:3 against 3:2) frame slightly differently across the width.

From Sensor sizes: Crop factor and field of view: Multiply a lens's focal length by the crop factor to get the full-frame equivalent view. A 35mm lens on APS-C frames like a 50mm on full frame. A 300mm lens on Micro Four Thirds frames like a 600mm. That's why smaller sensors are popular for wildlife: the same reach in a smaller, cheaper kit. The focal length converter does the sums for any pair of formats.

Quoted from this site's pages.