A slipping transmission usually refers to a vehicle's transmission that doesn't engage very easily or very readily. When this happens, you will shift the car from park to drive or reverse, and the gears may whine or the car may rev for a few seconds before it then suddenly lunges forward or backwards.
This often happens when the gears in the transmission begin to wear down, so that their teeth don't mesh together properly. The whining you hear is the metal rubbing against metal before those teeth fit each other; the car then lunges forward once the gears mesh.
This type of slipping is not a problem to overlook, as a transmission will not simply fix itself, and it will only get worse in time! If this is happening to your vehicle, definitely seek a professional for gearbox repairs. Note what causes this to happen and how to avoid it with your car.
Transmission fluid helps to lubricate those gears in the transmission system, so that their teeth don't wear down from constant wear and abrasion. Warmer fluid flows more easily and coats those gears more readily than colder fluid. Driving your car cold can then allow for more wear and abrasion against the gears, and for the transmission to slip sooner than it should. Give your car a few minutes to warm up every time you start it to protect all its parts under the hood, including the transmission.
Changing the fluid
The transmission fluid won't burn up or evaporate, but it does get old. Over time, it will pick up sediment and debris that are circulating in the transmission system, and that sediment then rubs against the transmission gears, causing damage to their teeth. Change the fluid as often as recommended in your car's owner's manual, so it's always fresh and clean and free of that damaging debris.
Low fluid pressure
For the transmission fluid to circulate around the transmission system, it needs to be pumped and pushed through that system. If the pump that creates this force in the transmission fluid tank reservoir begins to fail, the fluid may not circulate so easily. In turn, those gears may get worn down more readily.
There is also a filter that works to clean transmission fluid; if the filter gets clogged, the fluid may not move so readily through the transmission system. Your mechanic can test the fluid pump and change the filter, and note if this allows enough transmission fluid to flow through the system and keep the gears from slipping.