When a product is labeled "flushable" it generally means that it will clear the toilet bowl. It does not mean it will definitely clear pipes or break down in the sewer system or at a wastewater treatment plant. Water and wastewater utilities around the world have found a significant increase of wipes in their sewer pipes and at their plants.
Truly flushable items are ones that: break into small pieces quickly, are not buoyant, and only contain materials which will readily degrade in a range of natural environments (like paper, not plastic).
The word "fatberg" combines the words "fat" and "iceberg" to describe the masses of congealed grease and personal hygiene products that can linger in sewer systems. This mix of personal hygiene products and grease can create "fatbergs" in our sewers.