The Memrise algorithm predicts when any given word or sentence is likely to fall out of your long-term memory and it's time for you to review it. This is based on a learning technique called the spaced repetition system, which leads to stronger memories.
When reviewing a word or sentence correctly, the item will be moved to a longer interval. The review schedule is as follows:
Next review in: 4 hours > 12 hours > 24 hours > 6 days > 12 days > 48 days > 96 days > 6 months
If you get an item wrong during a review, it will be moved back to the first interval (i.e. to be reviewed in 4 hours).
You can see when specific words are due to be reviewed by visiting each individual level on the website.