Can I filter a function created by a theme or a plugin?

Filters are one of the two types of Hooks.

They provide a way for functions to modify data of other functions.

You can filter any function that has applied a filter on it’s variables during the function definition.

For example, in the get_bloginfo function, you would see a filter applied on it’s output just before the output is returned:

$output = apply_filters( 'bloginfo', $output, $show );

The above apply_filters call means you can modify the output by defining a filter such as:

function wpse253559_define_filter( $output, $show ) {

    return 'altered';
}

add_filter( 'bloginfo', 'wpse253559_define_filter', 10, 2 );

This would alter the output of get_bloginfo to always return “altered” no matter what it’s initial value is. You can read more on add_filter and apply_filters.

References:

• https://developer.wordpress.org/reference/functions/add_filter/
• https://developer.wordpress.org/reference/functions/apply_filters/