Your car battery is charged by the alternator when you drive. If your car is stationary for a long time, for example, because it's a classic car or convertible that you only drive in the summer, the battery will slowly drain. There's a good chance that the car won't start after being left standing for a long time. If your battery is completely drained, it may not be possible to recharge it at all.
To prevent the battery from draining, you can use a battery charger or trickle charger. This prevents your battery from going flat. If the battery is still good, but doesn't have enough charge to start your car, you can use jump leads or a jump starter. Many modern chargers and jump starters even work as power banks for other devices, such as your phone. This makes them incredibly useful to have around.