It commemorates the unity between Britain and America during World War II and the fact that over 1 million US soldiers and cargo/supplies passed through the Port of Liverpool on their way to take part in the D-Day Landings.
I also believe it commemorates the Port Battalions of the US Army Transportation Corps who were based in Liverpool and worked as stevedores, winchmen, signalman, riggers etc alongside their UK counterparts. Many of those serving in the Port Battalions would have been part of the D-Day Landings and gone on to work in recaptured ports in mainland Europe or taken part in the Pacific Theatre of operations.