Loitering seems to have incredibly vague definition, according to Wikipedia. It seems that it's illegal, even if you don't seem to be doing anything wrong. In the US, there are signs to this effect, which came to a surprise to me. Can I actually get into trouble if I, well, loiter, whatever loose definition you use?
Photo is from personal archive.
It depends what you're doing, where you're doing it, and (from personal experience) whether you look like a member of $racial_minority.
The loitering laws are in place so that the police can ask you for ID if they think you may be doing something illegal. For example, someone "loitering" on a parking lot can be looking for cars to break into. However, if you're stopped by an officer in the parking lot, show them your ID, and then tell them that you're looking for your car because you're not used to such immense parking lots, not only will all be fine, they may even help you figure out where you parked.
A Florida lawyer explains that
You can charged with loitering or prowling, if you are in a place and at a time where normal, law-abiding citizens are not commonly found and which could be construed as posing a threat to property or to others. If you are discovered by a police officer who asks you to identify yourself and you refuse, or you try to conceal your presence, or you immediately flee the scene, the officer may have grounds to charge you with loitering since you did not adequately justify your presence at the location.
Thus, as long as you can identify yourself, you should do fine as a tourist, since you can always justify your presence with "I'm so glad to see you, officer. I'm totally lost". Note that won't work quite as well if you're in a tent where they don't want you to be camping
In America, they don't want people to do anything that disrupts commercial activity. So begging is "out" for example. And "loitering" would be seen in a similar light.
There are signs at many establishments that read something like: "Patrons only, violators [or "loiterers"] are subject to arrest.
If you are in a store, you should be doing business with the store (or look like you are). Stores, and even libraries don't want people sleeping on the premises, or otherwise taking up space if you are not there for the stated purpose of the premise because they want to cater to genuine "customers." That also applies to places like construction sites.
In Nevada, it is a felony to use a computer in a casino to calculate odds (thereby reducing the casino's advantage). Not exactly "loitering," but it does illustrate a point about the "pro-commercial" nature of the American legal system.
Yes, you can be arrested for this; you usually won't, but usually isn't always, and you won't know until it's too late. The entire point of such laws existing in the first place is to give the authorities a legal justification to be able to arrest someone, harass them, or leave them alone, whichever they prefer, at a whim, so long as the target is within the specified location.
Note that begging, being homeless, and in some cases even just looking like you're really poor, are all common primary targets for such laws, because passing a "no loitering" ordinance and then enforcing it on anyone who looks homeless (and on no one else) is much better for public relations than a "jail time for being poor" law, so if you are going to be in such a place, avoid looking like you might be homeless, as it will substantially increase the odds of arrest.
You can wander about anywhere at anytime of the day or night without raising the slightest suspicion simply by taking along a dog.
