When you install Windows 10 (even with Windows 8 or later) or setup a new Windows 8, 8.1 or 10 PC, as one final part of the setup you will see the a similar image to the one below prompting you to store your documents on OneDrive by default.
I strongly suggest that you don't use this preference. This setting synchronizes all your files up to Onedrive "cloud" storage. which creates unnecessary congestion especially on devices that are used on a cellular (mobile network) connection. Additionally depending on how many files you have you will probably get the Onedrive is full error message and "do you want to buy more space?"
If you would like to use Onedrive for online storage/backup this can be a good idea but to do that you probably want to selectively sync some folders only.