Why do people put "m" in front of their instance variable?

public class Forum {
  private String mTopic;

  public Forum(String topic){
   mTopic = topic; 
  }
  public Forum(String topic){
   mTopic = topic; 
  }
  // TODO: add a constructor that accepts a topic and sets the private field topic

  public String getTopic() {
    return topic;
  }


  /* Uncomment this when you are prompted to do so
  public void addPost(ForumPost post) {
    System.out.printf("A new post in %s topic from %s %s about %s is available",
            topic,
            post.getAuthor().getFirstName(),
            post.getAuthor().getLastName(),
            post.getTitle()
    );
  }
  */

}

public class User {
  private String firstName;
  private String lastName;
  // TODO: add private fields for firstName and lastName

  public User(String firstName, String lastName) {
    this.firstName  = firstName;
    this.lastName = lastName;
    // TODO: set and add the private fields

  }
  public String getFirstName(){
    return firstName;
  }
 public String getLastName(){
   return lastName;
 }
  // TODO: add getters for firstName and lastName

}

public class ForumPost {
  private User author;
  private String title;
  private String description;

  public ForumPost(User author,String author, String description){
    this.author = author;
    this.title = title;
    this.description = description;
  }

  // TODO: add a constructor that accepts the author, title and description

  public User getAuthor() {
    return author;
  }

  public String getTitle() {
    return title;
  }

  public String getDescription() {
    return description;
  }
}

public class Main {

  public static void main(String[] args) {
    System.out.println("Beginning forum example");
    if (args.length < 2) {
      System.out.println("Usage: java Main <first name> <last name>");
      System.err.println("<first name> and <last name> are required");
      System.exit(1);
    }
    /* Uncomment this when prompted
    Forum forum = new Forum("Java");
    // TODO: pass in the first name and last name that are in the args parameter
    User author = new User();
    // TODO: initialize the forum post with the user created above and a title and description of your choice
    ForumPost post = new ForumPost();
    forum.addPost(post);

// */
  }

}