Your bitcoin are not stored in your phone or your wallet. They exist only as entries in the shared ledger. What you hold is the right to move them, and that right is a key.
A secret number
A private key is a number chosen at random from a range so large it is hard to imagine: about 2 to the power of 256 possibilities, more than the number of atoms in the visible universe. Two people picking a key at random will never pick the same one.
A key that signs
When you send bitcoin, your wallet uses your private key to produce a digital signature on the payment. Every node can check that the signature is valid, without ever seeing the private key itself. A valid signature is the only thing the network asks for.
▶ SEE IT IN THE MACHINEIn the WALLET, the private key is sealed inside the safe and only comes out as a signature.