I see syntax quit similar to this very frequently:
Sometimes it is associated with an assign statement:
assign S0 = 2'b00;
I tried searching online however I could not find any sources.
This is simply how a value is given.
4'd0 is a 4-bit value, given in base 10 (decimal), with value 0.
2'b00 is a 2-bit value, given in binary, with value 0.
The following four values are equivalent and all equal to fifteen:
4'b1111 (binary) 4'd15 (decimal) 4'hf (hexadecimal) 4'o17 (octal)
These are integer literal constants defined in section 5.7.1 of the IEEE 1800-2017 LRM. Anyone using Verilog/SystemVerilog should have a free copy available at https://ieeexplore.ieee.org/browse/standards/get-program/page/series?id=80