Being "vocal" means that you talk openly about your opinions. For example, this might mean disagreeing with a decision:
I tried to get my team to start practices earlier, but the players were very vocal in letting me know that they didn't like that.
Being "vocal" in some situations is a good thing. For example, some teachers might want their students to "be vocal" in class:
You're doing a great job, but I'd like it if you were a little more vocal.
Or a manager might want her employees to be vocal in meetings.