Symmetric cryptography uses one shared secret key for both encryption and decryption. It is fast and efficient, which makes it the workhorse for encrypting large amounts of data such as disks, files and network sessions. The Advanced Encryption Standard (AES) is the dominant symmetric algorithm today; it is a block cipher that works on 128-bit blocks and supports 128, 192 and 256-bit keys. Older symmetric algorithms you may see named include DES, which is broken because its key is too short, and 3DES, which is deprecated. Symmetric ciphers can also be stream ciphers, such as ChaCha20, which encrypt data as a continuous stream.
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