Small intro before latest blog posts

Two methods:

Edit the template file:

Find the template file that renders your posts (most likely index.php) and find the place where the post loop starts.

Something like:

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>

Any content added before this code will be shown before your blog posts.

Insert via hook:

add_action( 'loop_start', 'wpse107113_loop_start' );
function wpse107113_loop_start( $query ){
    if( $query->is_main_query() ){
        echo "<p>Hello World!</p>";
    }
}