A “special counsel” often refers to a lawyer from outside the government appointed by an attorney general or, in some cases, Congress to investigate a government official for misconduct while in office. “Cannon” might be a judge or other legal official involved in this case.
“Jury instruction” refers to the guidelines given by the judge to the jury about how to proceed with the case details, including the rules they have to follow and the standards they must adhere to when deciding if the accused is guilty or not.
A rebuke suggests that the special counsel has criticized or expressed sharp disapproval at these instructions. This could be due to a variety of reasons – a perceived bias in the instructions, a perceived error in the instructions, or perhaps a disconnect with the special counsel’s understanding or interpretation of the law.