There are many flavors, but consider the brain in a vat thought experiment:
You are sleeping one night, when the scientist takes out your brain, puts it in a vat, and connects its neurons to a machine that gives/receives electrical inputs such that the brain activity is identical to certain possible real life scenarios if the brain was still in the body.
The brain in the vat could be having the same experiences I am having right now, so why can I be confident that I am not a brain in a vat?