Flash message also known as popup or toast messages are used to communicate with users and to inform them of important changes or errors that has occurred while they interact with record forms.
To set up a new flash message that is not in the "out-of-the-box" installation, you need to:
Create a source message.
Create message translations.
Configure the business rule whose conditions trigger the message display.
Create a source message
All flash message texts are stored in the Source Messages (source_message) table.
To create a new flash message, complete the following steps:
Navigate to System Localization → Source Messages.
Click New and fill in the Message field.
Click Save or Save and Exit.
The text of the source message must be in English.