Each song page shows a level - Beginner, Intermediate or Advanced - and, under Difficulty score, a number from 1 to 10 such as "3 / 10"; the song lists show the same number beside the level. Both are estimates. Knowing what the number is made of tells you how far to trust it, and what you still need to check for yourself.
How the difficulty score is calculated
It starts from the three-level rating each piece is given: a beginner piece starts at 1, intermediate at 4, advanced at 7. Then comes note density - the number of notes divided by the length of the piece, or notes per second - which adds a point at each of 3, 5 and 8 notes per second, up to 3 points.
A piece whose range spans more than 76 keys gets one more point, for the shifts and leaps across the keyboard that no note count shows. The total stops at 10. In practice beginner pieces land between about 1 and 4, intermediate between 4 and 8 and advanced between 7 and 10, with the levels overlapping at the edges.
For example, La Candeur from Burgmüller's 25 Etudes (beginner, about 6.5 notes per second) scores 3, and the first movement of Mozart's Sonata K.545 (beginner, about 9.6) scores 4. Among intermediate pieces, the C major Prelude from Book 1 of the Well-Tempered Clavier (about 3.8) scores 5 and Invention No. 1 (about 6.6) scores 6.
What the score cannot see
Density is an average over the whole piece, so a mostly slow piece with one fast passage scores lower than it plays. Independence between the hands, awkward chord shapes, ornaments, pedalling and voicing are not in the number at all. A slow piece with a low score is not necessarily easy to play well.
So treat a one-point difference as roughly the same, read the level first, and use the score to compare pieces within a level. For a piece you are considering, listen to it with the falling notes and look for the things you personally find hard.
Using the browse pages
The pages for each level list the pieces from the lowest score up, so you can work down the list. On a category page, set Sort by to Easiest first to see a collection you like in the same order, and a composer's page lets you compare the level and score of each of their pieces.
On a smaller keyboard, choose "Fits 61 keys" or "Fits 76 keys" under Keyboard on the song list to narrow the list to the pieces that stay within it (a piece whose range could not be read stays in the list). Each song page also gives the range under Range, in octaves and keys.
Choosing the next piece
A rule of thumb: pick something at the same score as a piece you can play, or one higher, and let it bring one new thing - a different metre, a new key, a left hand that turns from accompaniment into a melody. One new thing per piece keeps progress steady.
If unsure, try a few minutes at 0.5x, hands separately, on a short loop. If each hand feels within reach by the end of that session, so is the piece. And when the order is the question, the practice paths lay out pieces towards a goal, with what each one teaches and how to practise it.