"getfield" part is to strip out the ports from the domain part, and redirect the user to it. For example, if the HTTP Host header is user will be redirected to http://www.domain.com.
There is a condition that tests the HTTP Host header equals to www.website.com. It means, if a user is browsing to http://www.website.com:80/uri or http://www.website.com:443/uri, the iRule will redirect the user to http://www.website.com/uri only.
However, if the domain is [HTTP::host] will likely to be www.website.com:8443 and it will fail the test, and users will not be redirected.