They're very simple reports meant only to monitor large transactions. Of course, since the value that triggers a currency transaction report hasn't been updated in decades, many more of these forms are generated than originally intended.
There's no need to panic if you find out one has occurred when you took money out to buy an electric car or put a down payment on your new home. They're routine and don't reflect on you poorly as long as you're honest.
Structuring and currency transaction reports
The hard-and-fast rule of thumb is that currency transaction reports are generated on transactions of $10,000 or more -- but that's not really the whole story. They're also generated when someone makes a suspicious transaction, especially if it looks like they're trying to purposefully get around a currency transaction report being generated.
This is known as "structuring." Structuring can trigger a currency transaction report, even if the amount of a single transaction is very low. For example, let's say you sell an RV for $30,000 in cash and want to deposit it without arousing suspicion. You might consider dividing the money up into six transactions of $5,000 each to avoid the report being generated.