What is the source of this verse? Also what does it signify? Does this mean that Sri Krishna is first Jagadguru or something else entirely?
It appears to be from Shree krishnaashtakam , 8 slokas eulogising Sri Krishna.
vasudeva sutam devam kamsa chaanuura mardhanam | devakee paramaanandam krishnam vande jagadgurum ||
It means that salutation to Sri Krishna, who is teacher for the world
Sri Krishna was one of the greatest yogis of his times.
It doesn't mean that there were no such yogis before him.
Sri Krishna himself mentioned about Usana in gita.