WordPress at a glance
function is not described

next_posts() WP 0.71

Displays or retrieves the next posts page link.

No Hooks.

Return

String/null. The link URL for next posts page if $echo = false.

Usage

next_posts( $max_page, $echo );
$max_page(int)
Max pages.
Default: 0
$echo(true/false)
Whether to echo the link.
Default: true

Code of next posts: wp-includes/link-template.php VER 5.1.1

<?php
function next_posts( $max_page = 0, $echo = true ) {
	$output = esc_url( get_next_posts_page_link( $max_page ) );

	if ( $echo ) {
		echo $output;
	} else {
		return $output;
	}
}