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What are scale degrees?
The notes of a numbered from the : 1 is home, 2 is the next letter up, and so on to 7. A ♭ in front (♭3, ♭5) means that note is one lower than plain.
Scale degrees are a way of naming notes by their job instead of their letter. Call the root 1 and count each letter-name above it: in A, B is 2, C♯ is 3, D is 4, E is 5, F♯ is 6, G♯ is 7 — those plain numbers are the major scale, the do-re-mi scale everything else is measured against. A lowers one of them by a half step, so ♭3 in A is C rather than C♯, and ♭5 is E♭ rather than E. The point of the numbers is that they travel: A blues and C blues use different letters but the very same degrees, 1 ♭3 4 ♭5 5 ♭7, which is why players think in numbers and can move a lick to any key.