Begin is used to denote the start of a block of code. A block of code can be zero or more lines of code. Blocks of code are used to gather the workflow of thought together. This means, if I am doing a comparison and the result is true, I could group the workflow for this in a block of code. Also, when you define a procedure of function, it's instructions are in a block of code between begin and end.

// Implementing workflow if A is 1 and B is 2:

   if (A = 1) AND (B = 2) then begin
      Writeln('Starting a workflow');
      ConnectToDatabase;
      UpdateSomeInformation;
      DisconnectFromDatabase;
   end

Begin
   writeln('Copyright notice here');
   if paramcount=0 then Begin
      showHelp;
      Halt;
   end
   else Begin
      showMainMenu;
   End;
end.

Source: Reserved Words

See Also:

end, else.