what is the meaning of message here and why we use it??
it means that if an exception ocurrs, it will log a message with the name of the exception, for example when you use a try catch in the catch activity you create a variable, this variable will alow us to access to the error properties like source, message etc…
normally we use it to know which is the error in the exception handling. Thi ex variable is the type System.Exception
Besides reading the documentation, I also suggest doing the free classes on the UiPath web site.
These are basic things it will teach you.