{n{expr}}
means create a concatenation by repeating the expression n times. $bits(Bus) is the number of packed bits of the data type of Bus.
You can simplify this expression with
assign (strong1,highz0) = Bus = En ? Data : 'z;
'z, '0, '1, and 'x are all extended to the proper width based on the context of the assignment target. In this case, Bus.