The idea here is that you are supposed to focus on one of these strategies. A firm should decide which strategy it wants to pursue and pursue only that strategy. This helps a firm to focus on its strengths and put all its energy into one strategy.
Porter's generic strategies for competitive advantage include cost leadership, differentiation, and segmentation. In other words, you can keep your costs and prices low, make your product special compared to others on the market, or focus on a specific market niche.