Adding from what others already said, conceptually, you go from blue to white.
Blue → yellow 1 → yellow 2 → yellow 3 → white underlined 4 → white.
If you don’t do anything with a blue word, once you page or finish a lesson, the system will turn it to white.
Otherwise, you click on a blue word and you choose or search for a definition, and becomes a yellow 1.
From there, you progress at your choice to move it to yellow 1 until white.
SRS system, that I don’t use, also move this progression calculated on how many times you worked on the same word.
Hope it helps.