Channel letters are a type of signage using embossed letters, numbers, or any other readable character. To make them, a sign manufacturer uses metal or plastic. Between these materials, plastic is a more favorable option since it makes installation not heavy.
Have you ever been to Google's headquarters' before? If you've seen their building, imagining what channel letters actually look like isn't hard. As signage, channel letters clearly indicate the name of the company in front of a building without worrying about obstructions.