On some SE sites, there is a tab called "Subscriptions" with a title of "Filter Subscriptions". What is this, and how does one use it? Example.
I did find a link that clears it up a bit: http://stackexchange.com/filters
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It lists all the subscriptions to tags you have.
When you hover a tag shown under a question, you see a popup similar to the following one.
When you click on subscribe, you get emails about new questions tagged with that tag, and probably about new answers to those questions.
In your case, you are subscribed to the questions tagged c#, and you get an email every day (in contrast with an email every 3 hours, or an email every 15 minutes).