What is this operator called as "+:" in verilog
That syntax is called an indexed part-select.  The first term is the bit offset and the second term is the width.  It allows you to specify a variable for the offset, but the width must be constant.

Example from the SystemVerilog 2012 LRM:

logic [31: 0] a_vect;
logic [0 :31] b_vect;

logic [63: 0] dword;
integer sel;

a_vect[ 0 +: 8] // == a_vect[ 7 : 0]
a_vect[15 -: 8] // == a_vect[15 : 8]
b_vect[ 0 +: 8] // == b_vect[0 : 7]
b_vect[15 -: 8] // == b_vect[8 :15]

dword[8*sel +: 8] // variable part-select with fixed width