Market neutral means that the strategy will make money in any market conditions. The market could be going up, down or sideways and the strategies returns are not decided by the direction of the market. We measure market neutrality in terms of beta. You will see beta and then a numerical value in the back tester. A beta of 0 means that the strategy is totally independent of the market, but that is very hard to achieve in practice. Quantopian accepts a beta of between -0.3 and 0.3 for its portfolio, and I usually aim for -0.1 > beta > 0.1 in mine. A negative beta is generated when the strategy loses money when the market makes money.
Topics to look at from here include beta hedging and otehr risk management topics.