Some fridges have a small drain hole inside the main compartment at the back. This lets any condensation or water created through the defrost cycle drain away. If it becomes blocked with small pieces of food or ice, water won’t drain away as it should.
This can often be sorted yourself. Try cleaning out the drain hole to let the water out. Cotton buds are good for this, as long as you don’t drop them in by accident. Some models even come with small tools especially for cleaning- check the manufacturer’s instructions if you are not sure.
If this doesn’t work, it could be a faulty part and will take a little more investigation- time to call for a hero.