The narrator explains that the half-life of a substance is the amount of time it takes to lose half of its initial value. In nuclear physics, it's how much time it takes for unstable atoms to lose energy by emitting radiation. In love, the narrator says the half-life is how long it takes for lovers to feel half of what they used to. The narrator says that when Natasha thinks about love, she thinks that nothing lasts forever. She believes that love has an immeasurably small half-life and when it's gone, it's like it was never there.
Natasha admits at several points throughout the novel that Rob was her first boyfriend, and she also purposefully kept him at arm's length. This shows that Natasha's habit of keeping people away makes her feel as though actual connections with people don't actually exist, given what she thinks about the half-life of love.