What is fungus on digital camera lenses and what is the remedy?

You had the camera and/or the lens stored away in a damp or humid place, maybe inside a dark cupboard in a dark room, causing fungus to grow inside the lens on the glass.

You could try putting the lens in sunlight for a few days to see if it kills the fungus and dries it up. If not, then you'll need to take it to a qualified camera technician and have them dismantle the lens, clean it, and rebuild it. This is, of course, an expensive thing to do.

You cannot remove fungus from any optical device...
better to take care not to allow its growth by keeping a beg of silica gel in the case with the camera, away from moisture... don't leave such devices in a car left in broad day sun shine... lest moisture would seep in and fungus would grow...