This is the chance that given a specific guid, that you'll find a collision for it. Utterly minuscule chance. However birthday paradox controls, if you generate 2^62.60 guids the chance that you've generated a collision is around 99%. Still enormously unlikely, but way smaller than 2^122.

At a rate of comparing 400,000 guids per second, you have a 99% chance of seeing a collision within the next 553,750 years.