Like other editors, Emacs has commands to search for occurrences of a string. Emacs also has commands to replace occurrences of a string with a different string. There are also commands that do the same thing, but search for patterns instead of fixed strings.
You can also search multiple files under the control of
(see Searching and Replacing with Identifiers) or through the Dired A command
(see Operating on Files), or ask the
grep program to do it
(see Searching with Grep under Emacs).