I have inherited a code base. I am having trouble identifying the significance of some of the flags used in make file. I am new to c++ prgramming

g++ -std=c++11 -fopenmp -O3 -g -o bfs main.cpp bfs.cpp ../common/graph.cpp ref_bfs.o

Specifically, what is the meaning of -03 flag and why do we need the last ref_bfs.o part.