This could mean a number of things. It could, for instance, mean that Bannon has already been cooperating with Mueller's team and has provided valuable leads and evidence. Even cooperative witnesses are routinely subpoeanaed.
Being called to testify before a grand jury almost certainly means that Bannon himself is not facing any possible charges. To do otherwise would violate Department of Justice guidelines. But it could possibly mean that a sealed indictment has already been lodged against Bannon and that he's cooperating and testifying much like former National Security Advisor Michael Flynn, who is now also assisting Mueller's team in their investigation.
Last night, The Daily Beast broke the story (link here) that Bannon will not be barred by executive privilege when testifying before the grand jury, something he evoked when speaking with investigators for the House of Representatives yesterday. "Mueller will hear everything Bannon has to say," a source is quoted as saying.
Given that Trump now loathes Bannon and caused his ouster from Breitbart, the hard-right conservative and nationalist website, Bannon has no reason to be loyal to the President any longer. Arguably Trump's downfall would benefit the former strategist in a number of ways as he makes future career moves.
This subpoeana could also mean Bannon is being called to corroborate evidence and testimony Mueller's team has already secured through other sources and witnesses. In the new Michael Wolff book that has the President so angered, Bannon speaks candidly about possible Trump crimes of which he has direct knowledge, notably money laundering.
Whatever the answers, without doubt this is bad news for Trump. The more people close to him who are questioned under oath, the greater the risk to him and his family.
If he has anything to hide, he has to be very nervous now. Stay tuned.