An email has been delayed
Before a message is sent it will undergo several checks. The mail server will determine if the sender is authorized to send the message. The spam prevention software will determine if the message can be considered spam. The message will enter the server's mail queue and will wait for it's turn to be sent. Then, the same steps will be followed by the recipient's mail server when it receives the message.
All of this happens very fast and a message is usually delivered to the intended recipient in seconds.
Certain technical difficulties could delay a message. Most commonly this is caused by a network issue with the communication between the sender and recipient servers.
If the delay is caused by a server problem, you will be provided with an error message which you can report to your host afterwards.