A private key is the alphanumeric character string that allows Bitcoin to be owned by someone on the blockchain. The private key is required to spend or move bitcoin funds from a Bitcoin address or wallet.
The common phrase, “If you don’t own your private key, you don’t own your Bitcoin” does a good job expressing the importance of knowing your private key. It is extremely important to keep your private key safe. If someone accesses your private key, they do not need your permission to take your Bitcoin.