SBC is SuBtract with Carry. It's like SUB, but it also subtracts one if the carry flag is set. It's the only way to do 16-bit subtraction/comparison since SUB can only operate on A.

Subtract with carry. It subtracts DE and the carry flag.

SBC is usually used when you need to do subtraction with 4 bytes and such.
;we will subtract big num 1 from big num 2

ld hl, (bigNum2) ;hl = $0201
ld de, (bigNum1) ;DE = $0605
or a ;this time we don't want the carry flag
sbc hl, de
ld (result), hl ;DE was bigger than HL, so the carry flag is set

ld hl, (bigNum2+2)
ld de, (bigNum1+2)
sbc hl, de ;since the last operation carried, (remember 2nd grade math)
ld (result+2), hl ;we had to subtract an extra 1 from this result

bigNum1:
.db 5, 6, 2, 3
bigNum2:
.db 1, 2, 8, 9
result:
.db 0, 0, 0, 0

This is what SBC is used for. But, we are forced to use it because SUB HL, DE doesn't exist.

I noticed that I have been ninja'd, but I don't care.