how to not show plugin in admin area

To complement @s_ha_dum’s answer: If you just want to offer a function to be used in a theme, make it a custom action.

Sample:

add_action( 'call_hello_world', 'hello_world' );

Now, in a theme, the author can call your function with …

do_action( 'call_hello_world' );

… and the function will print its content only where the author needs it.

This has at least three advantages:

• You can disable the plugin anytime, and nothing will break. If there is no callback for an action, nothing will happen.
• You don’t have to check with function_exists(), which is always … ugly.
• Another plugin can use the same action, or it can replace your callback.

Here is a sample plugin:

<?php
/*
 * Plugin Name: Custom Action
 */

add_action( 'call_hello_world', 'hello_world' );

/**
 * Demo function.
 *
 * Usage:
 * do_action( 'call_hello_world' );
 *
 * @wp-hook call_hello_world
 * @return void
 */
function hello_world()
{
    print '<h1>Hello World!</h1>';
}

// static class method
add_action( 'call_static_method', array ( 'Demo_Class', 'static_method' );


// dynamic class method
$demo = new Demo_Class;
add_action( 'call_dynamic_method', array ( $demo, 'dynamic_method' );

class Demo_Class
{
    public static function static_method()
    {
        print 'I was called statically.';
    }

    public function dynamic_method()
    {
        print 'I was called with an instance.';
    }
}

Install, activate, and use the do_action() in a theme as often as you need it.