WordPress is a free and open-source content management system written in PHP and paired with a MySQL or MariaDB database. Features include a plugin architecture...
There is most likely a mismatch between the content’s encoding and the encoding declared by your website. Check that the page is served with UTF-8 in the HTTP header and that the HTML declaration matches. Save your source files as UTF-8 too.
If only text retrieved from the database is affected, check the character sets used by the connection and the tables. Before converting anything, export the database and test the change on a copy. Changing the encoding declaration will not automatically repair text that was already stored incorrectly; you may need to correct it or import it again from the original source.