Docs is part of the free Office suite that is available to everyone with a Google account. It is a free word processor that you can use to create documents, letters, flyers, reports, etc.
Because the Google office system is online in the Cloud, you can easily create, edit or share documents on any device that has an internet connection. This means you can work on the train, or in the car while travelling. It also means that because the document is saved in the cloud, it is always saved, so the version is always the latest version, no more worrying if you are on the correct version of the document. Another advantage is that you can easily share with other people that will allow a team to work on the same document at the same time without any loss of data, and Docs keeps everything organised.