The shell of an egg is porous, meaning that water can pass through it. Over the egg incubation period, water passes from the egg to the air outside. Correct humidity ensures that the right amount of water is lost over time. It is important to remember that humidity will vary, and this is not a big problem; it is the average humidity that matters more.
A replacement Foam Humidity Pad, for the R-Com 20 range, will ensure correct humidity is maintained.