What is the key distribution problem of the symmetric (secret) key encryption?

When encrypting content with a symmetric key, the receiving party needs to have this key in order to decrypt. However, there is a fundamental problem that the key itself cannot be easily shared with the receiver over an unsafe channel, where attackers could eavesdrop and capture it as well.

The solution for this is either to use asymmetric key encryption, or to use a key exchange algorithm such as the one devised by Diffie-Hellman.