Every card detected in your notes has a so called “recall level” associated with it which indicates how well you remember it. It also represents the number of days after which it will be queued for review again.
When you are testing yourself, the recall level changes based on how well you remember the card. The theory is simple - if you remember it well the recall level goes up, otherwise it goes down.
Recall uses a simple algorithm and multiplies the current recall level by a constant and there’s a constant defined in your configuration for each of the review results. By default, these constants are 2, 0.5 and 0, which in practice means that
if you click Remembered, the recall level will be doubled;
and if you click Struggled, the recall level will be halved;
if you click Forgot, the recall level will go back to 1 (minimum value).