A private key is a string of zeros and ones that is usually 128 or 256 bits long. A bit is one 0 or 1. 128 bits therefore means a sequence of 128 zeros and ones in a row. Importantly, a Private Key is randomly generated.
Bitcoin works with so-called asymmetric cryptography. Somewhat simplified, your Wallet creates a Private Key and several public keys. The public keys in this case are used as receiving addresses (you may share them with others so they can send crypto to those addresses). Your Private Key is yours and you should never share it with anyone. Because only with this Private Key can the ownership of the crypto sent to the public addresses associated with this private key be transferred. This Private Key is a easily stored in a format that is understandable to people, namely 12 to 24 words. These words are called the Seedphrase, backup phrase or the backup words.