Meeting invitations look a lot like any other email message. The difference is that they control what appears on your calendar. So don't ignore or delete them. 

You'll notice that a meeting invitation includes buttons for

• Accept
• Tentative
• Decline
• Propose a New Time

Click the button that matches your intent, then choose whether to send a response. 

The meeting will appear on your calendar (unless you've declined it) and the meeting invitation will be automatically moved to your Deleted Items folder.

Notes

• If you don't send a response, the meeting organizer won't be able to tell whether you're attending. They'll see your status as "none."
• If you click Decline, the meeting won't appear on your calendar. So if it's possible you might attend, click Tentative instead.