Yes, it is true, to a point! It would be incredibly easy for Joe Bloggs to hook up to your wireless connection and use it to get free internet access, but it is trickier to get access to your pc. But it can be done and you should be aware of that.
Your wifi can be made secure very easily by using a number of steps.
Take your time to read this article and get to grips with it:
http://www.practicallynetworked.com/support/wi reless_secure.htm
If you are stuck, we need to know:
OS - Windows XP, Vista or something else?
Router make and model.