Symbolism, Imagery, Allegory
Love Potion Number Nine? Not quite: Tristan and Yseut don't go around kissing everything in sight. They just go around kissing each other.
Either way, the love potion is the most famous aspect of the story of Tristan and Yseut. It's a symbol the story uses to convey Tristan and Yseut's powerlessness to resist their love for one another. As the great literary critic Ke$ha tells us, their love is like a drug. The love potion makes them willing to sacrifice anything to be together, including their worldly comforts and social status. So the love potion might be a symbol for the way love is this big all-consuming thing, or for the way nothing matters to the lover more than the beloved.
One question we might ask is whether the love potion is real or more of a metaphor. Did Brangain really mix things up, or was the accident in any way intentional? Since fate is a big element of this story, couldn't Brangain's mistake be fated? Yseut's mother cooked up the love potion so that Yseut could be happy with her new husband. Maybe the point is that human emotions can't be forced, that love will have its own way. Then again, maybe it was just a random fail.