This means +/- 1% of the full scale value, regardless of what point in scale a measurement is made. For example, on a meter with a 0 - 100 scale Volts DC, +/-1% of full scale is +/-1Volt DC. Then, even if you are making a mesaurement at 10, 25, 50, 75 or 100 Volts DC, the tolerance in that example would be +/-1 Volts DC.
When you add +/-1 least significant digit to the specification, this is added to the above tolerance. So in my second example, lets use the same meter as above (0-100 Volts DC), and we will say the least significant digit in the display is 1 Volt DC. When you add the +/-1 from the first example to +/-1 digit, the total tolerance is now +/-2 Volts DC. so regardless of what reading is involved, the total tolerance using +/-1%FS +/-1 LSD, is +/-2 Volts DC.
Tolerances for the above readings would be 10 +/-2VDC (8 - 12 VDC); 25 +/-2VDC (23 - 27 VDC); 50 +/-2VDC (48 - 52 VDC); 75 +/-2VDC(73 - 77 VDC); 100 +/-2VDC (98 - 102 VDC).
If the resolution on the meter had been 0.1 VDC, the total tolerance would have been +/-1.1 VDC.