Salisbury is an English nobleman. In the beginning of the play, he is loyal to King John, and he's one of the nobles who stands flanking him in his courtroom. He also plays the role of peacemaker and intermediary to the other side; for example, he tries to calm Constance down when she is grieving about the coming marriage between Blanche and Louis.
In the second half of the play, though (beginning roughly in Act IV), Salisbury changes his tune—and also becomes a much more important character. That's because he is one of the nobles who confront King John about his strange behavior in getting crowned for a second time, and he also requests that John release young Arthur.
When King John reveals that Arthur is dead, Salisbury is furious. He and Pembroke storm out of the king's presence, determined to find Arthur's body and get to the bottom of what's been going on. When they do find the body, Salisbury quickly steps into the role of ringleader. He's the one who first kneels before Arthur's body and swears allegiance to it. He's also the one who nearly picks a fight with Hubert because of his outrage at the young child's death.
Salisbury shows his independent spirit later on, once he and the other English lords have joined forces with Louis. There, in what doesn't seem like the best time or place for it, Salisbury defies expectations by making a long speech about how steamed he is to have to join forces with a foreigner (Louis) in a war against his home country.
As it turns out, Louis appears to admire the speech—or, at least, he tells Salisbury that he does. In fact, Louis is actually planning to have Salisbury and the other rebel English lords executed once the war is over. That's just how these politicians roll in Shakespeare's plays.
When Salisbury and his gang learn that Louis's going to betray and kill them, Salisbury is, once again, the one who takes the lead, suggesting that it's time for the nobles to swear allegiance to King John again. He's even at King John's side when the monarch dies. So even though Salisbury is definitely a minor character, he has a distinctive personality that makes him stand out from the crowd.