What is Edge Lettering on Coins?

By GovMint :

Edge lettering is a technique that involves placing letters or symbols on the edge of a coin rather than on its obverse (front) or reverse (back). Edge lettering on coins can function as an extra security measure or an aesthetic or stylistic choice. Examples of modern U.S. coins that include edge lettering are Presidential Dollars which have inscriptions such as the date, mint mark, and mottos, "E PLURIBUS UNUM" and "IN GOD WE TRUST," adorning their edges.

Sources Used:

https://nnp.wustl.edu/library/dictionarydetail/515813