Expressions are basically a list of sentences that define the parent intent. While the intent represent a generic description, expression define what users would actually be saying to express it.
As an example, a "Hotel Search" intent could contain the following expressions:
Hello, I'm looking for an hotel near the beach
I need a room with ocean view
I've been looking for a room for 2 persons
I want to book a room
I need a room for 4
As you add expressions to your intent you improve the training of your AI and improve the future confidence percent of user messages. The more expressions you add the more precise the AI engine will become after training.
Expressions alone may be enough for some situations but most of the times your chatbot will need a way to retrieve specific dynamic content from an expression to trigger specific flows. This is done using Entities.