If you have 5 firms with ROIs of -10%, -5%, -3%, 1%, 14% and 31%, then 50% of the firms have negative growth rates and the median firm loses money.

However you can observe that if you invested $100 ($600 total) into each firm, you would end up with $636.

Median return isn't a very useful measure of the health of a sector. It is a predictor for risk, but not a very good one.