The plugin takes a Start value, and an End value, and will produce a unique sequence of those numbers, without repeating. The range is inclusive, so if you specify a range of 1 to 5, you will get the numbers 1,2,3,4,5, in a random pattern.
A Threshold value lets you configure how the cycle repeats when all of the numbers are used, and the sequence needs to continue/cycle. A Threshold=0 will completely ignore the previous sequence, so the numbers will be random within the same range again, but it is possible that the last number of the sequence may be the same as the first of the next. For example; 2,5,4,1,3 could be followed by 3,1,4,2,5. The value 3 will repeat because the sequence must be regenerated and no logic is applied to test the last value of the last sequence. A Threshold=1, would guarantee that the first number of the next sequence would not be a 3, so there would not be a repeat immediately when the sequence restarts. Care is still required not to abuse the Threshold, as if it is too large (that is, greater than half the range), it is impossible to guarantee not repeating the same values.