Once you have created your first bot, you will be asked to create your first message - the welcome message. The is the message the user sees if they access the bot from the Get Started button. It isn't always the first message the user will receive, as you might decide to intro users to the bot from a broadcast message etc. However, it is good practice to have a welcome message in place that explains what the bot does, just in case people come across the bot organically.
Here is a video to help you build your welcome message: