Symmetric Cryptography means that the key used for encryption is identical to the one used for decryption. This means that in order for two parties (Alice and Bob, as an example) to communicate, they need to share this key somehow.
Using purely symmetric cryptography, this is not possible. In order to send the key, one would need to encrypt it. For that, a key is necessary, and the problem begins anew.
How can symmetric keys be exchanged?
One way would be to use asymmetric encryption. This can either mean using an algorithm which generates a shared key, or by simply encrypting a key and sending it to the other.
Another possibility is out-of-band exchange, meaning that keys are exchanged through a different method than is used for communication. This could mean in person, via letter, on the phone, over the radio, etc...
None of these would be practical for certain ways of communication. For example, communication with a web-server would be completely impractical if I had to call them beforehand for them to transmit a 128-bit AES key to me over the phone, and it would suffer from the same problems as we had initially, since that call would not be encrypted.