This selects the CPU to tune for at compilation time by determining the processor type of the compiling machine. Using -mtune=native will produce code optimized for the local machine under the constraints of the selected instruction set. Using -march=native will enable all instruction subsets supported by the local machine (hence the result might not run on different machines).
"-mtune -O2" are CFLAGs.
"+dvd +restricted-codes" are examples of USE flags
I do not know whether or not the original poster knows the difference, so it is entirely possible that your response is fine.