A digital synthesis flow is a set of tools and methods used to turn a circuit
design written in a high-level behavioral language like Verilog or VHDL into a
physical circuit, which can either be configuration code for an FPGA target like
a Xilinx or Altera chip, or a layout in a specific fabrication process
technology, that would become part of a fabricated circuit chip. Several digital
synthesis flows targeting FPGAs are available, usually from the FPGA
manufacturers, and while they are typically not open source, they are generally
distributed for free (presumably on the sensible assumption that more people
will be buying more FPGA hardware).