Condition: jumps if SF = 1. SF is simply a copy of the top bit of the last result: bit 7 for a byte operation, bit 15 for a word. Read as a signed number, that means the result is negative. MOV does not set flags, so to ask about a value already in a register use TEST AX, AX or OR AX, AX first.

Prints Y.
MOV     AX, -1234
TEST    AX, AX   ; flags from AX itself: SF = 1
MOV     DL, "Y"  ; MOV leaves the flags alone
JS      SHOW1    ; taken: DL stays "Y"
MOV     DL, "N"
SHOW1:
MOV     AH, 02h  ; DOS: print the character in DL
INT     21h

SF is reliable as "the top bit is set", but not as "a < b" after a compare: if the subtraction overflowed, the sign is wrong. For signed comparisons use JL / JGE, which also look at OF. A handy non-arithmetic use: SHL AL, 1 moves bit 6 into the sign position, so the next JS tests that bit.

Range

On the 8086 JS exists only in the SHORT form: two bytes, opcode 78h 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.

JNS     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 88h followed by a 16-bit displacement (32-bit in 32-bit code), so from the 386 on JS can reach anywhere in the segment. Code for the 8086 must stay with the short form.