Thats just a metric to determine how scrambled the signage points you are receiving are.
A sub slot has 64 signage points.
If the sub slot goes from 1 → 64 signage point continuously, it counts it as a complete sub slot,
if it’s missing one signage point, or there’s a badly ordered sequence, (e.g.: index 3, 5, 4) it counts it as incomplete sub slot.
It doesn’t mean your incomplete sub slots are not useful, it just means that you could, in theory, have missed a signage point from that sub slot, which in turn could mean that you missed a challenge.
If you hover over that progress bar below “128 sub slots” you’ll see that it says something like “89% ordered signage points”. You should not worry about this too much, this value fluctuates with the network. For example, when transactions were released in May, everyone had around 60% ordered signage points.
Hope this answers your question 