Explain the concept of port 80 blocking.

Regarding the port 80 blocking - In short: USUALLY, keep it open and use it to redirect everything to HTTPS.

Now onto the complicated stuff: taking away port 80 can stop cookie-thieves that passively look for straggle http://corp.com/some/forgotten/thing requests. The TCP connection does not succeed, the browser does not send the GET and cookies, and the bad guy cannot read them. Sometimes this is a reasonable thing to protect against, especially thinking of corporate environments: legacy apps, HSTS only partially implemented, cookies that may lack the secure flag or path or host restrictions, third-parties hosted or proxied, ...

Now, should you block it? Probably not.

Like others mentioned, that would complicate setting up Let's Encrypt and prevent redirects (including users that just type your.com in the address bar). If you have set domain-wide HSTS, removing redirects may even be considered counterproductive (you may want to risk one plain HTTP connection so it will protect all future ones). Also, note that active attackers will not be stopped (they can make the connection complete artificially, MITM proxy tools may even do this by default), there are corner cases (plain HTTP proxies, delegated domains outside your firewall), and you may just consider the passive attack too complicated for your model. Finally, should you add port 80 to a new server? Well, unless you already have a reason for opening it (see above), no.