It's actually pretty simple.

5 years ago, you bought 1000 shares of company X, and the "market value" was $50 per share, so your investment was valued at $50,000.

In the last 5 years, the market has increased, bringing the value of your investment to a theoretical $81,000 on February 14. I say theoretical, because you didn't sell, and you didn't get the gain.

Since Feb 14, the market has taken a short term drop, reducing your theoretical value to only $80,000. If you didn't panic and sell, you really didn't lose anything, because you still own the original 1000 shares.

With a cattle herd, if 20 cows die, they never come back to life, and the farmer takes a real loss. With stocks, unless you actually sell, or company X actually goes out of business, then you didn't really lose anything.

All calculations above are based on the actual performance of the Dow-Jones over the last 5 years. $18,132 on Feb 27, 2015, now $27,514 today.