Hyperventilation is when someone is taking in more oxygen than they really need. This causes an imbalance of the various gases in your body. Although it may sound good to get in more oxygen, in fact it is not, and can cause havoc with the breathing mechanism.
A person may hyperventilate because of anxiety, emotional stress, or by doing it on purpose by breathing too fast.
The person will be breathing too fast, they will look anxious, weak, tired, and they will become unconscious if the condition continues.
Help move the person away from the cause (e.g. watching an accident scene), talk and reassure them, have them sit down, ask them to breathe with you so they will slow down their breathing rate. If the condition continues then you need to activate the EMS because this can result in unconsciousness.