The 90% horizontal line is your Desired Retention. The card should be shown to you once the probability of you recalling that card goes below that 90 %.
The x-axis shows you the date. Whenever the line goes straight up to 100%, you have reviewed the card. That makes sense as the time you took and the effort you needed isn’t relevant for the forgetting curve; if you see the question and answer today, you will have a 100% probability of recall (how fast it decays again depends on several factors). This is based on cognitive sciences and psychology.
As an example: on approx. Wed 10, you reviewed your card and had a 100% chance of recalling it. A little bit afterwards your chance was at 90%. You didn’t review the card until late Thu 11, though. At that point, you only had a 60% chance of correctly recalling the card. After seeing the card, your probability of recall became 100% again. On Fri 12, the card crossed the 90% threshold and has been reviewed on Dec 14, after which the probability of recall was 100% again. And so it goes on .
