A subpoena is a notice from the court that they are going to exercise their authority to compel you to testify or produce evidence. Failure to do so will result in a penalty of some kind. In Mark 8:11 a coalition of Pharisees (and Matthew adds Sadducees as well) stood off against Jesus. They tried to subpoena him to produce evidence that he was really God.
They marched up to him and began to argue with him. Typically an argument has two sides. This case is no different. They demand a sign from heaven. They demand a grand cosmic affirmation that Jesus really is God’s Son. He responds by saying no. He simply dismisses the demand and further undermines their authority.
The scene is dramatic. The most influential fundamentalists link up with the most elite religious power-brokers in an effort to test a common enemy. The word “test” in Mark 8:11 is the same one used to describe the confrontation with Satan in the wilderness temptation. They put it all on the line because if Jesus doesn’t do what they demand, it proves he has no fear of them. Just like Satan in the wilderness they pulled out all the stops. The compromised every conviction they held in order to present a united front against Jesus to make him cave in. He doesn’t. He just says no. He just walks away. The encounter started as a trial for Jesus but ends in a verdict against his enemies.