Preconnect is a somewhat advanced feature to instruct browsers (<a href="https://caniuse.com/#feat=link-rel-preconnect">if they support it</a>) to make a connection to specific domains even if the connection is not immediately needed. This can be used e.g. to lessen the impact of 3rd party resources on HTTPS (as DNS-request, TCP-connection and SSL/TLS negotiation are executed early). Use with care, as preconnecting to too many domains can be counter-productive.