If you look e.g. in Hull's book in the section about hedging then you get more details.
You have a position of $X$ (thousand) Euros in a bond/basket of stocks and you want to hedge this position by shorting $N$ futures contracts that correlate to these instruments (Bund/Treasury futures or equity index futures).
You hedge because you think that there is some difficult time in the next say week. You don't simply sell your position (the bond/the stocks) for various reasons (could be: you are not allowed to have too much cash in the portfolio, transaction costs are too high ...) so you short a derivative.
If the change in price of the derivative has little to do with your bond/stock then this will not be a good hedge but just another position. This risk is called basis risk.