Soundex
Soundex was a system invented in the USA after World War I to represent in print the way that a word is heard rather than the way it looks. It was important in indexing Census records before computer-based searches became common.
Words that are spelled quite differently have the same Soundex Code if they sound the same. For example Eberhard, Eberhart, Everhard, and Everhart are all represented by the Code E166. This was important to make census data handling manageable in a country with many immigrants from non-English speaking countries.
You can find more details on the history and early uses of Soundex in the Family Search Research Wiki
It is possible to calculate the Soundex for any name by following a set of rules as set out in articles like Finding your ancestors with Soundex from Ancestry Help.
Fortunately it is not necessary to do so, because computers can be programmed to do the conversion. There are several on-line forms that handle this task including:
There is continuing value in using Soundex while there is a backlog of files available as unindexed images on Family Search. Many of these (particularly those of US origin) are arranged in Soundex order. You can avoid flicking through thousands of images if you use Soundex to begin your search in the correct part of the film.