What is "Overlamping"?
It's when a light fixture is fitted with a light bulb that has a too-high wattage. Every fixture has a wattage rating that is recommended by the manufacturer.
Is it Dangerous?
For sure. Putting a 100-watt bulb in a 60-watt fixture could cause intense heat, melting the light socket and the insulation on the fixture's wires.
Any time you have that kind of damage on wires, you're at a big risk for arc faults, where an electrical current falls off its intended path— a leading cause of home fires.
Even after you pull the offending bulb out, you could still have lasting damage to your fixture.
How Can I Avoid It?
More modern fixtures should disclose their wattage rating—just don't go above that with your bulbs and you'll be safe. If your fixture is older, and without a wattage recommendation, go the safe route and use 60-watt bulbs (or lower).