It means that there is a clash. However, it is probably only with the numerous lab sessions or tutorials that are available and you need to pick one that is not clashing.
What does a Frowny Face on my timetable mean?
Do I have to go to all the lab sessions?
You only have to go to one lab session per course. This is usually the same for tutorial or workshop sessions. You need to choose which time you want and register for it after your first lecture. The lecturer will give you further instructions on how and where to register.
I have a timetable clash, what do I do?
What room is my class in?
Where do I get my timetable?
Go to the timetable page to work out your individual timetable http://timetable.anu.edu.au/