Every ROM comes with a kernel built into it. It's up to you if you want to update or change the kernel when running a ROM. The "kernel" is basically is the part of the OS that controls stuff running behind the scenes if you will. Visually, you'll see no difference in different kernels, but since they can control the amount of power and processing resources allocated to apps and the operating system itself, the right kernel *can* provide improvements to performance and battery life.
A swap file is used as "virtual memory", and setting that up could also help improve performance, but in my honest opinion, that's much more of an expert realm that installing ROMs and kernels, and therefore I would not even pay much attention or worry about that until you've mastered the basics.
After rooting, the first thing I'd recommend experimenting with is installing ROMs (and possibly radios, if needed). Once you're comfortable with that, you can look into different kernels. Then, once you really understand what's going on with the device (and possible have some understanding of Linux), you *MAY* want to dive deeper into things like swap files...but 99% of the folks around here that are rooted and running custom ROMs never mess that sort thing.