Travel to an area over 2500m (Cusco, Quito, La Paz, Kilimanjaro, Lhasa are some popular high altitude destinations) can put you at risk of altitude illness, caused by less oxygen in the air. Acute Mountain Sickness (AMS), high altitude pulmonary edema (HAPE) and high altitude cerebral edema (HACE) are risks that must be managed and prevented. The mildest of the the three, AMS, is still described by the 1993 Lake Louise Consensus Committee definition: a recent gain in altitude, several hours at the new altitude, and presence of headache and at least one of the following: gastrointestinal upset, fatigue, weakness, dizziness, lightheadedness, or difficulty sleeping. It is wise to consider these as signs of "AMS until proven otherwise," descend 100-300m and give your body time to acclimatize. If symptoms are getting worse while resting at the same altitude, one must descend as they are at risk of complications or death.
Increasing altitude gradually and giving the body a chance to acclimatize is the best way to prevent altitude illness that may hamper your travel or even prevent further ascent. Other important precautions include avoiding alcohol and caffeine, drinking lots of fluids, and bringing along medications which may prevent and/or treat altitude sickness.