Basically what IchigoSmof said. With Memrise, a new word needs to be "planted", then grow into a "flower" to count as learned. This usually is Preview card (seed planted), six questions that grow in how many multiple choices you have (leaves sprout), then you're learned (flower blooms). After that, words are slowly retested in a spaced pattern (4 hours, 12 hours, 24 hours, 6 days, 12 days, 24 days, 45 days, 90 days, 180 days).
The initial six questions can be quite tedious. If you get anything wrong during the learning, it extends how often you see it until it counts as officially learned.