How to migrate pages to new site

I’ve just built a new site and most of the content is new. However, there are a number of articles I need to migrate from the old site to the new one – is there a way I can do this via the file directories? Also, apparently these articles are one of the main ways that people discover our site and someone has told me that I need to make sure I 301 redirect all the old urls to the new corresponding urls – does anyone know what this means? ??

Thanks