A 301 Redirect Tells Google that the old location of a page or domain is now the new one (new domain or new page URL). It's important to add these so websites search engine results are not impacted
Add them to your .htaccess file or use a plugin. For only a few you're better to add them to the .htaccess file than using a plugin
Further reading

Setting Up a Permanent 301 Redirect in .htaccess

Domains
Redirect 301 / http://newdomain.co.uk/
Pages
Redirect 301 /oldpageurl /newpageurl