Stability is how long the memory survives, the half-life of your recall. It grows each time you successfully retrieve the card, and it grows more when the retrieval was difficult.
Difficulty is how resistant this particular item is to becoming stable. It nudges up when you press Again and drifts down when you press Easy, so intrinsically hard material keeps getting the extra exposure it needs.
The next interval is the point at which predicted recall falls to 90%. You will feel slightly unsure at every review. That feeling is the mechanism working, not a bug in the schedule.