The term backend as a service or BaaS is currently mentioned a lot among mobile application developers.
Multiple storage options exists for HTML5 applications, like the local storage or filesystem. But these will only allow application to store data locally, on the mobile device.
If your mobile app has to store data outside the device, in order to sync across different platforms for example, application creators have to make a decision on how this will be achieved.
One option is to build a simple backend service which could handle requests from mobile application running on multiple mobile clients and provide data storage functionality.
This usually sounds easier then it is to implement, because while building such backend service you have to take many factors into account. Authorisation and authentication, performance and operations support among many others.