Motorbike batteries are accumulator devices that are used to store electrical energy and transfer the energy when needed (for example when the starter button is pressed on the bike).
Most motorcycles and scooters that are regularly used (like bikes used for commuting) should not need to be charged up as they regularly receive a healthy charge whilst the bike is being ridden.
But, motorbikes and scooters that are not used very often (summer only riding, racing, fair weather riding etc) can suffer from starting and running problems if the battery is not kept topped up and maintained by a motorcycle battery charger.

There are many different brands and types of battery chargers, ranging from basic chargers for topping up to intelligent and smart battery chargers that are designed to be left connected to the battery for longer periods and can also help to recover deeply discharged batteries (as long as the lead plates and battery internals are not damaged).
Battery chargers that are designed for use on motorbikes and scooters are usually motorbike trickle chargers that use a very low charging rate (normally not above 1 amp/hour).

Please note – Due to the small size and complex nature of bike batteries, it is important to use a specifically designed motorbike trickle charger. General vehicle and car battery chargers are designed to charge much bigger and more powerful batteries and charge at a much higher rate. This higher charge rate can damage the lead plates in motorbike batteries and cause the battery to fail.