hi..
The threshold is the value based on the received bits level compared to which the bit is detected. If the received bit amplitude is less than the threshold, the bit is detected as a logic “0”, and if the bit amplitude exceeds that of the threshold, the bit is detected as a logic “1”.
but i also agree with naazira because the bits are taken to be equi-probable. i.e, bits 0 and 1 have equal probability of being transmitted