Each note in a specific key has a number assigned to it based on its position.
In the key of C Major
C is 1
D is 2
E is 3
F is 4
G is 5
A is 6
B is 7
So it is safe to say F is the fourth tone of the C major scale and Bb is the 5th tone of the Eb Major Scale.
Sometimes you might also come across Roman numerals (a.k.a Nashville Number System) which is the same thing too.
What are the benefits of understanding the number system?
The number system helps you learn songs faster and also allows you to transfer them to other keys with ease.
For instance, you could learn the song Mary Had a Little Lamb using the numbers instead of the Tonic Solfa Notation.
Ma-ry(3-2) had(1) a(2) li-ttle(3-3) lamb(3)
little(2-2) lamb(2), li-ttle(3-5) lamb(5)
Ma-ry(3-2) had(1) a(2) li-ttle(3-3) lamb(3)
its(3) fleece(2) was(2) white(3) as(2) snow(1)
Remember we learned this melody earlier in the key of C Major but now you can play this easily in any key with the numbers. Just pick a key like G Major and represent the G Major Scale with numbers in ascending order.
G(1) A(2) B(3) C(4) D(5) E(6) F#(7) G(1)
Now Mary Had a Little Lamb in Key G will be
Ma-ry(B-A) had(G) a(A) li-ttle(B-B) lamb(B)
little(A-A) lamb(A), li-ttle(B-D) lamb(D)
Ma-ry(B-A) had(G) a(A) li-ttle(B-B) lamb(B)
its(B) fleece(A) was(A) white(B) as(A) snow(G)
Now that you have learned the the number system it’s time to learn the scale degrees.
In music each number represents what we call scale degree and it’s defines the relationship between the 1st tone(tonic) and the other scale tones.
The one 1 is Tonic
The 2 is called Supertonic
The 3 is the Mediant
The 4 is the Subdominant
The 5 is the Dominant
The 6 is the Submediant
The 7 is the Subtonic or Leading tone
There will an in-depth explanation of what each of the scale degree names means but for now just try and remember the names.