Filter Hook

A filter hook in WordPress lets you intercept and modify data as it is processed. Unlike action hooks, which execute code at a specific point, filter hooks receive a value, let you modify it, and expect the modified value back.

Common examples include the_content (modify post content before display), the_title (modify post titles), and wp_nav_menu_items (modify navigation menu HTML). You attach a function using add_filter($hook, $function, $priority, $accepted_args).