g/LSB meaning in accelerometer measurements
In ADC or DAC terminology, one LSB is the size of one step that an ADC can resolve or that a DAC can output.  So you can have a one LSB step between 0 and 1, and one LSB step between 254 and 255, or anywhere else in the ADC range.
One LSB is the smallest that a single reading of the ADC can resolve, not measure. - If an ADC is noisy, then you may not be able to measure down to 1LSB in a single measurement, but if it's noisy and more accurate than its ADC, you may be able to average multiple measurements below one LSB.
Basically, the 10 bit resolution of the ADC affects your reading because, for a perfectly linear 10-bit ADC, there are \$2^{10} = 1024\$ steps between its minimum reading and its maximum reading.  A \$\pm\$2g accelerometer with a 10-bit output is going to have 1024 different possible measurements from -2g to +2g, so -- depending on some fine details in how they treat end points -- one LSB will be exactly, or close to, \$\frac{4\mathrm g}{1024}\$.
As a counter-example, if it were a 16-bit output, then 1 LSB would be \$\frac{4\mathrm g}{65536}\$.