By default you will receive a digest summarizing recent changes in a project. You can control the frequency of this email, including switching it off completely, by clicking your name at the top right of the screen and going to Profile, then scrolling to Email Settings. The dropdown menu controls the maximum number of times per day you will receive a digest email, ranging from 0-4 times. This is a maximum setting and in most cases you will receive fewer.
You can also choose which events you would like to be alerted to, such as uploads or new comments. Any which are deselected will not appear in the digest. These settings also affect which emails you will receive when subscribed to a file.
Switching off email notifications completely
To completely silence alert emails from a project you can mute it. To do this, go to the Projects page, hover over the project and click the bell icon with a line through it that appears. You can click the icon again to unmute it.
Preventing your actions from notifying others
If you need to do something that you don't want others to be alerted to, click your name at the top right and click Disable Alerts. An icon will appear next to your name to show alerting is disabled. This will ensure anything you do is completely silent and won't notify others.
This setting is remembered when you login, so remember to switch it off again when you have finished using it.