What is inside in $item in start_el function in Walker_Nav_Menu class

Take a look at the source of the class-walker-nav-menu.php, here.
There you can find the start_el method and in the commends just before the method saying that the $item variable is @param WP_Post $item Menu item data object.. So, it’s a post with post type nav_menu_item. You can have a look to the wp_setup_nav_menu_item function (here) which builds up a menu item.

The above is the background. To actually see the contents of an item, you can call
a filter in the start_el method and print the $item variable,
e.g.:

function what_the_item( $args, $item, $depth ) {

    // pretty print the contents of $item
    highlight_string("<?php\n\$item =\n" . var_export($item, true) . ";\n");

    // Because this filter will be called way more than one time,
    // in order to make the output cleaner, you can stop php execution
    // right after the print of the $item so only one $item will be displayed!
    // die();
    return $args;
}
add_filter( 'nav_menu_item_args', 'what_the_item', 10, 3 );