What is threadLocal variable?

ThreadLocal is a class. If a variable is declared as threadLocal then each thread will have a its own copy of variable and would not interfere with the other's thread copy. Typical scenario to use this would be giving JDBc connection to each thread so that there is no conflict.

ThreadLocal class by JAVA API
public class ThreadLocal {
  public Object get();
  public void set(Object newValue);
  public Object initialValue();
}

Implementation of ThreadLocal
public class ConnectionDispenser {
  private static class ThreadLocalConnection extends ThreadLocal {
    public Object initialValue() {
      return DriverManager.getConnection(ConfigurationSingleton.getDbUrl());
    }
  }

  private static ThreadLocalConnection conn = new ThreadLocalConnection();

    public static Connection getConnection() {
      return (Connection) conn.get();
    }
  }