S4S lumber stands for "surfaced four sides," and it refers to lumber that has been planed smooth on all four sides. This type of lumber is typically used in applications where a smooth, finished appearance is desired, such as in furniture making or trim work. 

There are a few key benefits to using S4S lumber in your projects. First, it is generally easier to work with because it has a smooth surface that with a little sanding is ready for finishing. This can save time and effort in the woodworking process, as you don't have to spend as much time sanding or planing the wood yourself. Additionally, because the wood is already smooth, it is easier to get a uniform finish on your project, which can be important if you are trying to achieve a specific look or color. Finally, S4S lumber is often more expensive than rough-sawn lumber, but it may be worth the extra cost if you are looking for a high-quality, finished look for your project.