Condition: jumps if ZF = 1 or SF ≠ OF. After CMP a, b, ZF = 1 means a = b and SF ≠ OF means a < b (see JL for why the sign and overflow flags are compared), so together they mean a ≤ b for signed numbers. It is the exact opposite of JG. The unsigned twin is JBE.

It fits the bottom of a counting loop written like "for i = 1 to 5", where the last value must still run:

Prints 12345. JL would stop at 4.
MOV     BL, 1    ; i = 1
NEXT1:
MOV     AL, BL
ADD     AL, "0"  ; digit -> character
MOV     DL, AL
MOV     AH, 02h  ; DOS: print the character in DL
INT     21h
INC     BL
CMP     BL, 5
JLE     NEXT1    ; while i <= 5

If the counter could start above the limit, test before the body as well, or the loop runs once anyway.

Range

On the 8086 JLE exists only in the SHORT form: two bytes, opcode 7Eh 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.

JG      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 8Eh followed by a 16-bit displacement (32-bit in 32-bit code), so from the 386 on JLE can reach anywhere in the segment. Code for the 8086 must stay with the short form.