first things first;
pc part picker can be used to put all the parts you want into a "build" to see if they're compatible, and there are also a variety of build guides there you can look at or just outright use if you don't feel like/don't feel confident about picking out your own parts.
overclocking is where you take a part and make it operate beyond the "stock" setting to produce more performance, usually at the cost of higher heat output i think? for the most part it's pretty safe to overclock as long as you don't ultra mega crank it.
i'm fairly certain, although it's not easy to clarify since i'm not much of a hardware person, that this is just saying it will support any DDR4 RAM up to 3466, and/or(?) it will allow you to overclock RAM to(/past?) that frequency.
since I'm not really sure, and I can't figure it out from a simple google search, i'd recommend checking it in pc part picker, and that ~should~ let you know about any incompatibilities the hardware pieces you choose have. if you still feel fairly uncertain, look at some of the build guides on pc part picker and maybe pick one of them, or check out
logical increments, which is a wonderful website that tells you a fairly good (maybe best for the price?) build that you can just grab all the parts for without having to think too much about it.