Figurative: "What was said is the absolute truth and can be verified by a third party source." Comes from an obscure reference to cheques (or checks, if you're American), in that such is a guarantee that you can take the document to the bank and redeem it for its face value.

Note the pronoun 'it' can be varied accordingly.

When you eat dirt for the first time and leave your family to build a shack in the woods