What is the review threshold?
At the bottom of your daily studies, you will find a section that displays the number of reviews waiting for you the next day. This is labeled Tomorrow's Reviews, followed by some numbers. You can see this in the screenshot below.
The first number is the number of reviews you have in queue for the next day, while the second number is your review threshold. The threshold is fully configurable by the student, so feel free to set an amount that you think will work for you, or simply stick with the defaults. This can be adjusted at any time.
Once you meet or exceed the review threshold, the system will stop you from moving on to new material so that you can review and better understand the information you have previously learned. At this point, the system will also refrain from assigning you a new unit for as long as your reviews for the current day or the next day exceeds this threshold. As your understanding increases, your cards for review will decrease below the threshold, thus telling the system you are ready to take on new information.
When both your current day's and next day's reviews are above the threshold, you will not be assigned any new units. This will be a 'review only day' and you will not be able to start any new material.
If only your current day's reviews are above the threshold, but your next day's reviews are below the threshold, the system will not assign you a new unit. However, if you feel like you can handle some new information after you've completed your reviews for the day, you can start new units manually until you meet or exceed your limit on the next day's reviews. Please note that the limit here is not a limit on the number of reviews you receive the next day. For example, if you have 200/50 reviews, those 200 reviews will still be queued for you the next day. While over the threshold, you will simply not be assigned a new unit for the day.
Once both your current day's and next day's reviews drops below the threshold, you will once again be assigned a new unit at the start of the day to do (after you finish your reviews of course), and you can proceed to do more units as long as your next day's reviews stays below the threshold.