What does Ex. 15:11 mean when it asks who is like you O Lord among the gods?

It means that YHVH, our Lord is not like any other. He is distinct, unique and “above all gods“. 
Ps 95:2-4 states:

Let us come before His presence with thanksgiving, Let us shout joyfully to Him with psalms. For the LORD is a great God And a great King above all gods, In whose hand are the depths of the earth, The peaks of the mountains are His also.…”
While other so-called “gods” may be real beings like Satan, the god of this world, (2 Cor 4:4), YHVH is superior to all these “gods” and He should be the only one to be worshiped, not others. However, when YHVH, the LOGOS, incarnated into man-Jesus, he said, “worship the Father”(John 4:22-24).