Milk teeth are the first of 2 sets of teeth that mammals get in their lives. The second set are adult teeth.
Milk teeth are unique to mammals.
Each special type of milk tooth (such as canines or molars) grows, falls out and gets replaced by adult teeth at roughly the same time in the upper and lower jaw. Because of this, the teeth can fit together precisely, and cut up food in our mouths more effectively when we bite and chew. This helps with digestion.
Having milk and adult teeth may only be possible in animals that also lactate (feed milk to their young from mammary glands) which is another unique feature of mammals.
Mammal teeth also have a special type of tooth enamel which is more durable and lasts longer.
The trouble with only having two sets of teeth is that once they are worn out, the animal can no longer feed itself properly. Once the second set of teeth is ground down, a mammal will die.
This problem has had an effect on the evolution of mammals in many ways. It also means that the shapes of mammal teeth are closely linked to the type of food they eat. Elephants are a good example of this.
Elephants today use their teeth to eat all sorts of plants, but their distant ancestors probably mostly ate soft leaves from trees and shrubs. A few million years ago the climate has changed and become drier, and grasses with tough leaves replaced a lot of the trees and shrubs.
Shifting to a grassy diet, the elephants' teeth began to wear down more quickly, and this gave a survival advantage to individuals with larger, longer teeth. They were the ones who lived for long enough to pass on more of their genes to the next generation, including the genes for larger teeth.
Over time, the proportion of elephants with larger teeth in the population as a whole increased, in other words, the teeth of elephants evolved.
Other animals with teeth grow them continuously throughout their lives. For example, the teeth of a reptile or a fish are gradually ground down by hunting and eating, but because they are always growing, the length stays the same.