Function: replace-string

Replace occurrences of FROM-STRING with TO-STRING.
Preserve case in each match if `case-replace' and `case-fold-search'
are non-nil and FROM-STRING has no uppercase letters.
(Preserving case means that if the string matched is all caps, or capitalized,
then its replacement is upcased or capitalized.)

If `replace-lax-whitespace' is non-nil, a space or spaces in the string
to be replaced will match a sequence of whitespace chars defined by the
regexp in `search-whitespace-regexp'.

In Transient Mark mode, if the mark is active, operate on the contents
of the region. Otherwise, operate from point to the end of the buffer.

Third arg DELIMITED (prefix arg if interactive), if non-nil, means replace
only matches surrounded by word boundaries.
Fourth and fifth arg START and END specify the region to operate on.

Use M-n to pull the last incremental search string to the minibuffer
that reads FROM-STRING.

This function is usually the wrong thing to use in a Lisp program.
What you probably want is a loop like this:
(while (search-forward FROM-STRING nil t)
(replace-match TO-STRING nil t))
which will run faster and will not set the mark or print anything.
(You may need a more complex loop if FROM-STRING can match the null string and TO-STRING is also null.)