The Jury consists of the final seven (or nine) evicted houseguests as members, not including the Final 2. Their purpose is to vote for the winner of Big Brother.
The Jury House
Members of the Jury are added to a special "Jury House" Facebook Messenger group chat where they can communicate with each other their decision on who to vote for or events that happened during their time in the house. They are given updates by the hosts on what's going on in the house by informing them of competition results, evictions, etc. The jurors are forbidden to communicate with the remaining houseguests and vice-versa.
On finale night, the finalists are given time to make their final plea as to why they should win. The jury gets to ask questions to the finalists to help them decide. Each juror then casts a vote for whom they would like to see be the winner of Big Brother. The votes are sent to the hosts and are eventually revealed in a forum post to show who the winner of the season is.