The thing at the bottom of Ikea coffee mugs is a drain called drainage gates.
The tiny hole at the bottom of the Ikea coffee mugs is to drain any excess water after the Ikea coffee mugs are washed.
These drainage holes or drainage gates are meant to transport any water away from the coffee mugs which would otherwise accumulate when the coffee mug is upside down in the dishwasher.
A small chip at the bottom of most coffee mugs is called a "kiln mark" or "firing pin mark."
The Kiln mark on the coffee mug is a result of the manufacturing process, where the mug is placed in the kiln and supported by small prongs to prevent the glaze from sticking to the kiln shelf.
A coffee mugs bottom is often not flat, but either concave or has an extra rim, to reduce the thermal contact with the surface on which a mug is placed.
These features often leave a characteristic circular stain on the surface.
And the handle of a mug keeps the hand away from the hot sides of a mug.