what 'internal' keyword will do?

Answer 1

Internal types or members are accessible only within files in the same assembly.

Answer 2

Internal is one of the 5 "Access Modifiers" and "Internal" access is restricted to the code in the same assembly.
Access modifiers are not allowed in namespace declarations; public access is implied for namespaces. The top-level classes (declared directly under a namespace) can be only public or internal. The internal access modifier is the default for a class if no access modifier is specified. One should use the most restrictive access level that makes sense for a type member.