Browsers fire a copy event when you copy, just like a click event fires when you press on a button. We just added analytics to it like we would any other feature on the site.
We didn't track the content of your copy (browsers don't let you see the text content) but we did track the following:
Meta data about the post and it's parent post like the id, owner, score, tags, if it was a question/answer, if it was accepted
If your copy was from a code block or from text content
The Referer header
Standard analytics properties like the date/time, approximated location, account metadata
Here's our privacy policy on analytics: "Analytics information Stack Overflow uses data analytics to ensure site functionality and to optimize our Product and Service offerings to you. We use web browser and mobile analytics to allow us to understand Network and Apps functionality. In doing so, we record information including, for example how often you visit the Network, how often you contribute content, Network and Apps performance data, errors and debugging information, and the type of activity you engage in while on the Network or in your use of our Products and Services. We may on occasion share this information with third parties for research or product and services optimization."