Occasionally I will see that symbol beside a component that I know is fully defined with mates. When that happens a Ctrl+Q rebuild has always fixed it.
For components that have their positions fully defined, but can still rotate (like a bolt in a hole), you can right-click on the Mates folder and select "Lock Rotation" from the drop-down. That will lock rotation on all concentric mates.
Similar to this, if you have a component with a Profile mate, you can also right-click on the Mates folder and get the option to lock Profile rotation (see the screenshot in the reply below from @berg_lauritz). However, I only get this option about half the time. If you don't see it, you still don't have to open the mate's property manager to lock it. You can expand the Mates folder directly under the component's file name in the tree and right-click on the Profile mate to get the same option, but of course it will only apply to that specific mate.
In a sketch: If you see (-) beside a sketch in the tree, in either a Part or Assembly file, it means that one or more of the elements in the sketch don't have their positions fully defined with relations and/or dimensions. By default, fully defined elements are black, and undefined ones are blue.
In most cases you should fully define sketches. I've heard of at least one case where the user didn't do that, so parts were made that were off just a little bit. It wasn't discovered until the parts had already been fabricated. It cost the company hundreds of thousands of dollars, and the user lost his job.
While you're in an active sketch, and it's fully defined, that symbol may still be there, but you can check the status bar at the bottom of your monitor. It will let you know when the sketch is fully defined, and if it is the symbol will go away when you close the sketch.
