Natural
Reasonable, Obvious, Usual, Common
noun, な adjective, の adjective
You've seen how 当たり means success. If you've experienced success before, everything after that seems to be pretty obvious.
The readings for this word are both kun'yomi readings. You've learned both of these readings by now so you should be able to read this on your own.
こんな当たり前のこと、どうして分からないの？
This is normal stuff, why can’t you get that?
当たり前のことだけど、毎日シャワーをあびることは大切です。
It's obvious, but it's important to take a shower every day.
他人にしてもらったことを当たり前のことだと思わずに毎回「ありがとう」と言いましょう。
Say thank you every time someone does something for you, without taking it for granted.