James, many thanks!

Adam, thank you so much! It's clear for me now.

It's similar to someone saying "Shut up!" to a friend, typically when that person says something very surprising and/or very positive. Example: "Hey Jenny did you hear what happened to Sofia? She went on a date with George Clooney." "Shut the front door!" Typically the expression would be used with someone who you are very familiar or casual with, and it might not work with everyone's personalities since it could come off as perhaps a bit rude to certain people. Edit: Also of note. It is typically used by women.

It's a kind of a joke or a euphemism for "Shut the f*** up!" People use it jokingly as a lighter version of that phrase with friends or in informal settings to express surprise, disbelief, or excitement.