When someone registers for an event, becomes a member, or makes a donation, automatic communications are sent not only to these people but also to you, the administrator, as you are copied on these emails.
If you no longer wish to receive confirmation e-mails when someone registers, you need to configure go to the settings for the feature of your choice. For example, for members, go to "Members" > "Settings".
Once on this page, click on the "Communications" tile, then click on the communication you wish to stop being a copy of. Then simply uncheck the "Administrators" box.
Participants will continue to receive automatic communications, but you will not.