A glimmer is a tiny glint of light or the sliver of an idea. Either way, it's a sign of a lot more going on behind the scenes.
A glimmer of light is just a little bit of light, maybe sneaking through the curtains enough to make a flicker on the floor. A glimmer of an idea is just a small inkling of an idea. When someone doesn't understand something, you could make them feel worse by saying, "You don't have a glimmer of what I'm talking about, do you?" When it comes to light, you can also say light is glimmering, or gleaming. People often say stars are glimmering.