It means that the adhesive does not have a good bond between the tiles and the substrate, or that there are hollow voids of missing adhesive under the tiles. If tiles are laid correctly and there is 100% adhesive coverage, and the bond is sound, then the tiles effectively become one with the substrate.
As the floor is used and subjected to foot traffic, the tiles that are not correctly laid will most likely break, crack, chip, or fail in some other way. In extreme circumstances, the tiles will actually lift off the floor.

Pro-Lay Wall and Floor Tiling understand It can often be less costly and time consuming to tile this way.
However, it will raise the floor height and that can create a step at door ways and other areas.
As long as the original tiles that are down are not drummy and were laid correctly.
We use a primer that will allow the tile adhesive to adhere to the original tiles and therefore will bond correctly and can be done.

Under the Australian Building Code and Australian standards (AS 3740-1994), you are required to have some form of waterproofing in areas of your home with plumbed-in devices. This includes kitchens, bathrooms, toilets and laundries. Not doing so may result in fines, but also increases your chance of water damage in your foundations.

Floor tiles are generally thicker, so they can withstand foot traffic and other impacts. Wall tiles are thinner and lighter to make them easier to adhere to the vertical surface.

You can tile over most wall and floor surfaces you find in your home including concrete, timber, brickwork, plasterboard and more. You can even tile over tiles!
Because each surface type has its own characteristics (such as rate of expansion and contraction, moisture absorption, 'dimensional stability,' and so on).
Some surfaces may require special preparation and different surfaces will require a different adhesive.