What Excel calls a chart layout is a combination of choices affecting such elements as the chart title, legend, axes, axis titles, and gridlines. Each of the program's chart types comes with an assortment of packaged layout options. With the chart selected, you can see the available layouts by clicking the Design tab under Chart Tools on the Ribbon and then opening the Chart Layouts gallery (use the scroll bar or arrows at the right of the gallery to see all the options). These are the available layouts for a line chart:
You can experiment with these preconfigured layouts by clicking buttons in the Chart Layouts gallery. If none of them gives you exactly the combination and positioning of chart elements (titles, legend, gridlines, and so on) you're looking for, don't worry. The Layout tab (to the right of Design under Chart Tools) changes the Ribbon so that it provides explicit control over each kind of chart element.