Each base quality is an ASCII encoded representation of that value Q.

So if you watched the optional videos just prior to this one,

then you already know what Q is.

But the short version is that Q is just an adjusted version of the probability,

which is called P that the base call is incorrect.

So when Q is higher, we can be more confident that the base is correct, and

when Q is lower, we're less confident that the base is correct.

And if you want to see more details about why we would use this particular scale for

queue, or how we would go about estimating the probability P,

you should watch the optional lectures that come right before this one.