Hypotension
If a person's blood pressure is low enough that they are feeling dizzy, they should lay down on the ground.  Putting their feet up, or bending their knees, is also helpful.  This makes it easier for blood and oxygen to get to the brain (because the blood does not have to work against gravity to get up to the brain, like it would if the person were standing).  Lying down will also make sure that the person does not hurt themselves by falling if they faint.