What does the 2h, 1d and 2d mean? Is that a bug?
No this is not a bug.
The buttons corresponds to your memory of the current card, e.g. if you think you know everything on the current card, then click “Easy”, and MN would schedule it to be reviewed in 2 days. Similarly if you feel you can’t remember much of the things on this card, click “Hard” and MN would schedule it to be reviewed very shortly.
This is a very popular and efficient methodology of learning new knowledge called “Spaced Repetition System” (a.k.a SRS).
This is very cool!!! Thanks for the explanation.