Symmetric encryption algorithms get their name from the fact that they use the exact same key from the encryption and decryption process. For this process to work both the sender and the receiver must have the same shared secret key to be able to perform the encryption and decryption of the data they are transmitting.
Symmetric encryption is also commonly referred to as secret key encryption, because for the communication between sender and receiver to remain secure it is important that the shared key remain secure, as you can imagine if anyone got hold of the key they would be able to encrypt and decrypt messages.
There are quite a few well known symmetric encryption algorithms, some are more common than others, the more popular methods are those which employ bigger key sizes, also their are three main types of symmetric encryption methods
Block ciphers
Message Authentication Codes (MAC)
Stream Ciphers
Symmetric methods compared asymmetric encryption methods are very fast and use up less resources on your devices and for these reasons they are used more often for bulk encryption when you need to ensure complete privacy.
The main drawback to this method of encryption is the initial and continual safe distribution of keys between your devices without the keys becoming compromised.
Nowadays a new method of key distribution is used where an asymmetric channel between two systems is created to build a secure tunnel over which your shared key will be exchanged before any important user data is transferred.