The function wp_remote_get() lives in wp-includes/http.php. WordPress does not load that file when the core is incomplete, when a script runs before the bootstrap, or when the file is corrupted. Wordfence calls the function during its initialization, so the missing definition triggers a fatal error.
The error stops Wordfence from scanning, updating, or communicating with its servers. It appears as a white screen or a critical error message in the admin area.