The yes command outputs the same string, STRING, in a constant stream. If STRING is not specified, the word it repeats is "y".
yes dates back to a time before Unix commands included the "force" (-f) option, which for many commands is the same as answering "yes" to all prompts.
yes can be used to do something very similar: if the output of yes is piped to a command, it will effectively answer "yes" to any confirmation prompts.

yes [STRING]...

yes [OPTION]

yes | rm -i *.txt

Removes?all files with the?extension?.txt?from the?current directory. Here,?yes?outputs a constant stream of "y"?characters, and that output is piped to the?rm -i?command, which prompts for confirmation before deleting each file. The?y?characters from the?yes?command will respond "yes" to each prompt automatically.

yes n | rm -i *.txt

Here, the?yes?command outputs "n" in a constant stream to the?rm -i?command, answering "no" to all the same questions. Therefore, this command deletes no files.