Have you ever experienced something that makes you wish it wasn't really happening? Well, the world is probably a computer simulation, which means that whatever is happening in the world around you, it doesn't matter. Because it's not real!
It's based on the idea that one of the following three principles MUST be true.
"The fraction of human-level civilisations that reach a post-human stage (that is, one capable of running high-fidelity ancestor simulations) is very close to zero."
A very mature civilisation would have enormous computing power – a much greater amount than we have now – and would be able to run simulations of their past (our present) without us being able to tell the difference. But in this instance, no civilisations achieve that state.
"The fraction of post-human civilisations that are interested in running ancestor simulations is very close to zero."
Alternatively, those civilisations have the technology, but are choosing not to run those simulations.
"The fraction of all people with our kind of experiences that are living in a simulation is very close to one."
Finally, if the previous two are not true, then all people living in a world like ours are actually living in a simulation.