How old is your dog? Does the knot move around, across the surface or is it deep down by the skull?
If your dog is older and the bump moves around a lot, and is kind of soft/squishy feeling (it's not hard, like a rock), it's *likely* a fibroma, a fatty tumor. These are *usually* harmless. If your dog is young, the knot doesn't move much or it feels hard, well, it could be more serious.
There is only one way to know for sure. As Anne says, bring your dog to the vet. Your vet will do what's called a needle aspiration (as long as the knot isn't so teeny tiny that the needle is too big for it), and will draw out some of the material of the tumor, which will then be sent to a lab for further analysis. The aspiration isn't as painful as it sounds, and you'll know exactly what you're dealing with. Fatty tumors often aren't a big thing. Older dogs get them often with usually no ill effects.
As Onyx points out, these tumors are actually cysts that can burst on their own, usually with no ill effects (although they're kind of gross when they burst), as long as we ensure that the open wounds don't then get infected. But often they just exist as lumps forever.
But depending on where they're at, a growth can be a problem just by its location, even if it's a simple "harmless" tumor. (Being situated by the eye does seem to me, as a layperson, to be one of those locations that might be problematic.) That's why I always bring my dog in. And sooner is better than later.
