Final answer:
The bolded word 'until' is used as a conjunction that describes a period of time during which the speaker could not understand the jokes, up to the point of seeing the show themselves.
Step-by-step explanation:
The word until in the sentence "Until I had seen the show myself, I couldn't understand the jokes." is used as a conjunction. It is used to indicate the time up to which the speaker was unable to understand the jokes. Essentially, it establishes a condition where the understanding of the jokes was not possible before a certain event, which is seeing the show, took place.