An arbitrage position typically comprising a long cash position together with a short position in its respective futures contract, whereby the cash price plus the cost of carry of the underlying position is lower than the futures price.
Arbitrageurs will therefore buy cash and 'carry' to the futures date for delivery into the futures contract. It is assumed that the cash position is financed in the overnight repo market. By convention, buying the basis is to buy cash bonds and sell futures, and selling the basis is to sell cash bonds and buy futures.