Ammonia basically burns your fish. Common signs of ammonia poisoning are fish resting on the bottom of the pond with their fins clamped tight to their body and sometimes a patchy white fuzz-like appearance.
You want a zero level at all times. If at any time your pond tests above 1 ppm, you should make a 25% water change --- adding a dechlorinator as needed. This should be done every other day until levels are below 1 ppm. Higher ammonia levels require greater/more frequent water changes. Feeding should be withheld or minimized, depending on the level. It takes 2-3 weeks in a new system for the bacteria in your biofilter to sufficiently convert ammonia into nitrite.
If you're showing an elevated ammonia level in an established pond it's probably due to one of the following REASONS:
1 - you have a build-up of crud (fish turds, leaves or other decaying matter) somewhere in the pond
2 - you haven't been monitoring your KH and your pH has crashed, killing your biofilter
3 - you're overfeeding your fish
4 - you're overstocked
5 - your fish have spawned
6 - you're underfiltered
1 - clean the crud out of the pond and maintain it a little better
2 - bring your KH up with baking soda
3 - feed them less
4 - get rid of some fish or build a bigger pond
5 - add an ammonia binder to detoxify the caustic effects and/or
perform a partial water change
6 - get a more efficient filter