ROS uses a simplified message description language to describe the data values that ROS nodes publish. With this description, ROS can generate the right source code for these types of messages in several programming languages.
ROS has a lot of messages predefined, but if you develop a new message, it will be in the msg/ folder of your package. Inside that folder, certain files with the .msg extension define the messages.
A message must have two main parts: fields and constants. Fields define the type of data to be transmitted in the message, for example, int32, float32, and string, or new types that you created earlier, such as type1 and type2. Constants define the name of the fields.
An example of an msg file is as follows:
int32 ...
