Essentially the private key is your password to access funds, it happens to be very complex because if not it would be easy to brute force, or guess, and retrieve access.
A private key (secret key) is a variable that is used with an algorithm to encrypt and decrypt code. Quality encryption always follows a fundamental rule: the algorithm doesn't need to be kept secret, but the key does. Private keys play important roles in both symmetric and asymmetric cryptography.
Bitcoin used SHA-256 and Ethereum uses KECCAK-256 for example. Both of these are secure public private key cryptographic schemes allowing users secure access to funds with their private key, and a public address to receive funds. The public address can be shared with anyone for transactions, but the private key should not be shared. Some applications, and wallets use seed words. Seedwords, are a form of a randomly generated private key that is easier to backup and human readable. While it seems counter intuitive seed words are secure, and generally consist of 14 or more words. There is enough randomization in the word list it can not be reverse engineered.