Chrome OS will save everything in the cloud. This means your documents and applications are not saved on your Chromebook, but on an external server. Because of this, your documents and applications will not use up any storage capacity on your Chromebook and therefore, your Chromebook will keep working at the same speed over time. By saving everything in the cloud you have access to all your documents and data from every device. The cloud makes it possible to log in to your Google Account from other devices and continue working on what you were doing. Because all your data is saved on an external server, you need a connection to the internet to gain access to your documents and data.
You can also save certain documents or applications on your Chromebook by enabling the offline use of this application. Please notice your Chromebook has a limited memory capacity which can get full when too many files are saved directly on your Chromebook.