>>724593604
>X
All you need to know is that there were two scientists who built robots and had them duke out. The rest you can surmise by just playing the games. For further context, you could watch the Day of Sigma OVA after playing X1 (the OVA was included with the PSP remake of X1).
>ZX
It follows the Zero series, but it's after a time skip with basically a new set of characters (biometals' appearances aside). You might not have all the context, but it's really nothing to worry about.
At the end of the day, these games are very gameplay-first, so while there's some neat bits of story here and there, it's not something to worry over too much.