What are watched records?
If you choose to watch a record, you will be notified whenever someone comments on the record. If you are not watching a record, you will only be notified if someone @mentions you in a long text field or comment box. For more on commenting and @mentioning, see the article titled "Comment on Records and @Mention Collaborators."
How do I watch a record?
To configure your watch settings, simply expand a record. The right side of the dialog is the record's activity feed. At the top right you can click the feed icon (which resembles an RSS feed or Wi-Fi icon.) Clicking this will allow you to change your watch settings between "Not watching" and "Watching comments."
If you comment on a record, or if another collaborator @mentions you in a record, you will automatically be set to watch the record in question. If you don't want to watch the record, you can just set your status to "Not watching" again.