Condition: jumps if SF ≠ OF. After CMP a, b you might expect the sign of a − b alone to answer "is a less than b?", but the subtraction can overflow, and then the sign bit lies. OF records exactly that case, so the CPU compares the two: when they differ, the true difference is negative and a < b.

After CMP a, bUnsignedSigned
a = bJE (ZF = 1)JE (ZF = 1)
a ≠ bJNE (ZF = 0)JNE (ZF = 0)
a > bJA (CF = 0 and ZF = 0)JG (ZF = 0 and SF = OF)
a ≥ bJAE (CF = 0)JGE (SF = OF)
a < bJB (CF = 1)JL (SF ≠ OF)
a ≤ bJBE (CF = 1 or ZF = 1)JLE (ZF = 1 or SF ≠ OF)

The example is chosen so that SF alone gives the wrong answer: −100 − 100 = −200 does not fit in a byte, the stored result is 56 (SF = 0), and OF = 1.

Prints Y. JS in its place prints N: the sign bit alone is fooled by the overflow.
MOV     AL, -100
CMP     AL, 100  ; -200 overflows: SF = 0, OF = 1
MOV     DL, "Y"  ; MOV leaves the flags alone
JL      SHOW1    ; taken: DL stays "Y"
MOV     DL, "N"
SHOW1:
MOV     AH, 02h  ; DOS: print the character in DL
INT     21h

Range

On the 8086 JL exists only in the SHORT form: two bytes, opcode 7Ch and a signed 8-bit displacement counted from the end of the instruction. The target must lie within −128..+127 bytes of that point. To go farther, invert the test and hop over an unconditional JMP, which can reach the whole segment. MASM 6 makes this rewrite itself when it finds the target out of range (its default OPTION LJMP), so the listing may show two instructions where you wrote one.

JGE     SKIP1    ; the opposite test, a short hop
JMP     FARAWAY  ; any distance in the segment
SKIP1:

Later CPUs

The 80386 added a near form of every conditional jump, 0Fh 8Ch followed by a 16-bit displacement (32-bit in 32-bit code), so from the 386 on JL can reach anywhere in the segment. Code for the 8086 must stay with the short form.