Ammonia is the highly toxic nitrogen compound fish excrete and that rotting food and waste release; in a healthy cycled tank it should always read zero.
Ammonia burns gills and skin and is the first killer in a new or crashed tank. It is the food source for the first group of beneficial bacteria, which convert it to nitrite.
A reading above zero means something is wrong — an uncycled tank, overstocking, overfeeding, a dead fish, or a filter that has been damaged. The fix is a water change now, then find the cause.