A joke is a display of humour in which words are used within a specific and well-defined narrative structure to make people laugh and is usually not meant to be interpreted literally. It usually takes the form of a story, often with dialogue, and ends in a punch line, whereby the humorous element of the story is revealed; this can be done using a pun or other type of word play, irony or sarcasm, logical incompatibility, hyperbole, or other means.
So I was curious so I found the joke. The joke was "before coming here, they news outlets said my meeting with Clinton would be a disaster. Now I can tell you that you're a disaster'
Boris is visibly drunk and audibly slurring in the video.