The one who owns the private key has the right to control the assets in the wallet. It means that if someone gets your plaintext private key, he will be able to control your assets. The private key kept in wallet is actually completely independent that can be generated and managed by the wallet without blockchain or network connections.
A BTC private key is a string of random digits. The control of the assets in a Bitcoin address depends on the ownership and control of the corresponding private key. For Bitcoin transactions, private keys are used to generate digital signatures which are required for payment, to verify the asset ownership. Private keys must be kept confidential, because exposing the keys to others is equal to giving the Bitcoin away to others. In case of loss, private keys should be backed up, as the keys cannot be retrieved once lost, and the Bitcoin it protects will be lost forever.