...I think it means the "Do Ray Me" changes from C to D or to E or to F, all the way to B. Would some song be "adapted"...
The term for this is transposition. You can transpose a song from one key like
C major to another like
...but cannot be played due to a missing black key on the piano? If any songs can be played in any key, how do the white keys and black keys make it always possible -- by what principle is it?
I don't know if "principle" is the right word, but all the piano keys - regardless of black/white color - in modern equal temperament tuning - are spaced one half step apart. Some systems like MIDI simply number those keys sequentially (1,2,3...) regardless of color. For example, MIDI key 60 is middle C. One way to think of transposition is adding a certain number of half steps to a given pitch. So, middle
C, scientific name
C4, MIDI key 60, can be transposed up two half steps by adding 2 to get MIDI key 62, or
D4. You could transpose it up three half steps by adding 2 to get MIDI key 63,
The point is theoretically there will always be a piano key available above or below because the piano keys are all separated by half steps and you transpose by some number of half steps. The color of the key may change, but nevertheless there will be a key available.
You can play around with this idea on the keyboard yourself to see it in action. Just play something simple like a
C major chord, or just the first five tones of
C D E F G. Then pick some number of half steps to transpose, like 3. Take each tone and move it up by 3 half steps. If you do that, the
C major chord becomes
E♭ major, and
C D E F G become
E♭ F G A♭ B♭. If you do that for transpositions of 1 to 12 half steps, you will transpose to all major keys, and you will see you never end up with any missing keys.
Simply put, you can transposed from any key to any other key. You will not find any missing piano keys.
I have seen some songs being played in F major or E major or C or D on the piano, and they correspond to different levels of difficulties.
Generally speaking, I think the "easy" keys are consider those with key signatures of up to two sharps or flats, or perhaps 3 sharps or flats. In a lot of beginner/recreational/practical music up to 2 sharps/flats will cover the bulk of material with 3 sharps/flats mixed in occasionally.
I think that sense of "easy" has more to do with reading the score than keyboard mechanics. For a beginner it's hard to remember all the sharps and flats of key signatures and keep track of accidentals. "Easy" keys just reduces the number of sharps/flats encountered in a score overall. When the beginner advances to playing comfortably in all keys, the notion of "easy" keys sort or goes away. Although for me, "hard" keys become hard, because of certain enharmonic note spelling and double sharps/flats that can be encountered.
It's interesting to note that Chopin considered the supposedly "easy" key of
C major a difficult key and keys with a few sharps/flats easier to play in, because the mixture of black and white keys fits the hand more comfortably.
Getting back to transposition. You could take some music that is relatively easy in one key, easy because the particulars of the piano key mechanics fit the hand, but when transposed into another key, it no longer fits the hand the same way, and it becomes hard. That is a possibility, but not a given. Lots of music can be transposed with no problems, and some musicians can transposed music on the fly.
For the SE community more than the OP, my answer is about tonal music, in major/minor key, in the typical range of typical beginner piano music. The OP is a newbie. They aren't asking about transposing in ancient tuning systems, or extreme ranges that would run off the ends of the piano keyboard.