Holding is something that just about all travelers will inevitably encounter at some point. I don't mean being put on hold when talking to airline customer service - I'm talking about "doing laps in the sky".
Holding is a maneuver that pilots are required to be proficient in so that when an arrival into an airport or specific airspace is delayed they can keep their aircraft within a confined area of the sky while not actually going anywhere.
When a pilot is given hold instructions by air traffic control, they are given a fix (a point in the sky defined by highly tuned instruments), which side of the fix to conduct the hold on, which direction to turn when established in the hold, and altitude to hold at and what time to expect to receive further clearance. Based on the aircraft's current position, the pilot (or some sophisticated auto-pilot systems) have to determine how to properly enter the hold and at what speed to conduct the hold at as holding speeds are highly regulated dependent on altitude.
To put it lightly, essentially all the airplane is doing is making circles in the sky. Let's look into why airplanes hold:
The most common reason for holding is an arrival delay. If your destination airport has had inclement weather move into the airspace surrounding the airport, planes then require even more space between them on approach which causes plane to "stack" up. Holding is used here to space out the aircraft so that the minimum distance between aircraft is maintained to a safe (and legal) distance.
If the weather happens to be a squall line quickly moving through, or a short thunderstorm (as is very typical at large airports like Atlanta, Tampa, Orlando, Ft. Lauderdale, Jacksonville, Charlotte, and Miami), arriving aircraft will be placed in a hold until the weather clears out. Usually a summer thunderstorm can clear out in as little as 10-15 minutes.
A less typical reason for holding could be if there is a disable aircraft on the runway, or some sort of occurrence that temporarily shuts down a runway. This is very rare.
The last one is the least common, and the vast majority of passengers will not only never experience this, but never even hear of it actually happening. If an aircraft experiences some sort of mechanical issue while in-flight and a diversion or air-return (industry speak for returning to departure airport) is necessary, often the decision is made for the aircraft to hold to either dump fuel, or hold so long that most of the fuel is burned off.
Now stick with me here, this doesn't mean there is impending doom or an extremely dangerous situation is about to occur.
Usually it is something as minor as overheated brakes or the flight computer saying there is a disagreement somewhere in the system, or an anomaly in the engine instruments. In these rare situations, the hold is used to burn fuel and for the pilots to discuss, brief, practice, and prepare what actions need to be taken. This is also the time where pilots can call mechanics on the ground and/or mechanics at the aircraft manufacturing facility to discuss the issue. Holding gives the pilots time to determine the safest course of action and to initiate those actions without being extremely pressured.
Holding is an operational necessity. In some cases it is ATC driven, in some it is pilot driven. In the end it is beneficial to all parties...and you can say you just played NASCAR in the sky.