Physical (haqiqi) impurities are substances considered impure by the Shari’ah. In other words, all things and substances are pure, unless the shari'ah says otherwise.
The basic ruling is that all substances are pure, unless there is evidence from the shariعah pointing otherwise.
Three common sources of physical impurities.
1. Coming out of the human body:
poo
urine
pus
blood
vomit
sperm
semenal discharge
2. Coming from animals or birds:
Blood that flows from an animal
The excrement of all animals
The droppings of all birds that can't fly
The flesh of dead animals and birds not slaughtered properly
The urine, milk and meat of animals whose meat is forbidden to consume
The saliva or sweat of pigs and dogs
The leather items made from from the skin of a pig
Droppings of birds that can fly but are considered forbidden to eat (weak impurity)
3. Alcohol related:
Wine made from grapes or dates
Other types of alcohol made for consumption
Synthetic alcohol (some scholars don't classify it as an impurity)
For the prayer to be valid, one's body, cloth and place of prayer have to be clean from these physical impurities. Therefore it is important to know how to remove them.