This causes fluid to form beneath the skin, causing a blister. As friction continues, more fluid forms, causing pressure and pain. Eventually, the blister may burst exposing raw skin, causing more pain.
Runners have problems with blisters usually at points where the shoe presses the foot too tightly.
Beginning runners get blisters, because they have not toughened their feet to resist the normal stress of running. Or their shoes may be new and not yet broken in. Experienced runners also have trouble with new shoes, or with old shoes that have become rough in places. If you get a blister, often you can take off your shoe and identify the point in that shoe that caused the problem. Maybe there is some imperfection that can be removed.