Looking for the meaning of "ad-hoc" in the dictionary helps with understanding what an ad-hoc report is:
ad-hoc: for a specific purpose; for the particular end or case at hand without consideration of wider application (an ad-hoc committee)
Consider some ad-hoc report scenarios:
Your boss gets a call from a doctor in Indiana. There was a fire in his office. All the consent forms for a trial need to be recollected. You need to generate a report of his patients in a particular trial and their addresses and phone numbers.
You get a call from someone that there is possible contamination of a particular lot number of contact lenses. You need to run a report that shows if the entire lot was shipped and if so, to where.
It's the middle of the month and it looks like funding for a particular project might be cut. You need to know exactly how many hours have been logged to that project since the last billing cycle.
An ad-hoc report generally answers some specific question that other reports (production reports) do not address. Sometimes, you can use a production report as the basis for the ad-hoc report. Sometimes you have to code the ad-hoc report for one-time use. Generally, with ad-hoc reports, you are not as concerned with the cosmetics of the report. The report might turn into a production report, but the usual expectation is that an ad-hoc report is tailored to answer one or two questions and answer them quickly (but correctly).