It means that the trade has earlier shorted and to the square of their positions, they had to buy.
Since there were so many short positions created in the market, people start buying, and that leads to the market going positive. One such situation is called short covering.
Short covering is a very peculiar situation where people start buying to square off their positions. Since so many people are buying, this creates a temporary rise in the price of the stock.
However, this price rise may not for a longer period. This price rise is because people are covering positions.
BUY or NOT
For that, we have to take into account 2 things:
A decrease in open interest and
A price increase.
So, a decrease in open interest along with a price increase mostly indicates short covering.