The wordpress theme contain a file called "footer.php" and that file contain function "wp_footer()"

The footer.php file is a base file of theme which generally contain a footer menu, social media widget, copyright text, javascript files etc

The wp_footer() is a wordpress hook used in footer.php template file to ensure that the right code is inserted (from the core/plugins/etc) into the right place. get_footer() is used in other template files to call for the code in your footer.php template file.

wp_footer()

This function does not accept any parameters.

Example: To add date in footer.

<?php
function yourFunctionName() {
    echo date('Y-m-d');
}
add_action('wp_footer', 'yourFunctionName', 100);
?>