like, if my readiness is at 82%, does that mean i have an 18% less chance of winning type of thing?
thanks

No, readiness only drops when you attack. It drops 4%-18% an attack depending on what % of your units sent(smaller % of units=smaller readiness loss). Readiness works just like your weaps so you multiply the break by readiness. If you have 1m turrets and 90% readiness, you only have 900k turrets. You gain 3% readiness back each turn used. While readiness drops to under 70% it caps there. So using the 1m turret example, 70% readiness or less is the same, 700k turrets.

Thank you. does this also affect when you are attacked, or only when you attack?

Like, let's say i attack someone and my readiness is 75%, then i attack again with 400k jets, i can expect to only have the effect of 300k. But when I'm attacked, does that mean only 75% of my turrets, troops, and tanks can defend?

That's right, readiness works for both attacking and defending. If you're at 75% readiness with 1m jets and 1m turrets, both will only be worth 750k.

I think missiles also drop your readiness (when you get hit by them) unless that changed, I'm not 100% sure though.