If you notice water under your AC unit, you may not actually have a reason to be that concerned. Some water under your outdoor AC unit can be normal in some situations. It may also be the result of a very mild problem. Rarely, it could indicate something big is wrong. So, it's important to look a little closer and deduce which of the following reasons is to blame for the puddle.
On a mild day, your AC unit won't produce much condensation because the temperature difference between the coils and outdoor air is not that significant. But on really hot days, the difference in temperature is greater, so more condensation will form. The condensation may form faster than the condensate drain line can drain it away, which will cause a puddle to form under your AC unit. So, if you only notice this puddle on really hot days, there's likely nothing to worry about.
Blocked Drain Line
If the fluid is clear and it's not terribly hot out, there's a good possibility your AC unit's condensate drain line is blocked. This line is meant to collect condensation produced by the AC unit, but if it is clogged, the water will just fall onto the ground and form a puddle. This is more of a nuisance than anything else. The air conditioner will still work, but the water may pose an issue because it can cause damage to things around your air conditioner. Have an HVAC contractor come out and take a look. They can either remove the blockage or replace the condensate drain line, neither of which is hard for them to do.
If you look at the puddle from a distance, it might look like the fluid is clear. But you should dip a piece of paper towel into the fluid to be sure. If the fluid is, in fact, light red or green, then it's not condensation. It's refrigerant from your AC coil. Wash your hands, and don't touch the refrigerant. An HVAC contractor will come to clean it up, locate and repair the leak, and recharge your AC unit with more coolant.
A puddle beneath your AC unit is not always a major concern. There's a good chance it's just condensation, although you should check and make sure it's not refrigerant before making any assumptions.
Contact a local AC service to learn more.