In Feser’s own words (from the third of the links I posted above):
Something is “determinate” in the sense in question here if there is an objective fact of the matter about whether it has one rather than another of a possible range of meanings – that is to say, if it has a meaning or semantic content that is exact, precise, or unambiguous. It is “indeterminate” if it does not, that is to say, if there is no objective fact of the matter about which of the alternative possible meanings or contents it possesses.
I’m going rewind our conversation slightly, because I did not explain things as well as I could have…
So, what is going on here is Feser is using the way that a material triangle deviates from perfect triangularity as an illustration of the fact that material things are not “exact” or “determinate” in the way that concepts are; unfortunately I think the illustration obscures the actual point somewhat.
Here is the actual point, which I was trying to explain: say you want to physically represent the concept “triangle”. You may try to do so by drawing a triangle. But nothing about the physical facts of such a drawing would fix whether it represented “triangle” as opposed to “triangle with these lengths and angles” or “2-d geometric figure”. You could add more instances of triangles to your drawing, but it still wouldn’t be determined by the physical facts alone whether it represented “triangle” as opposed to “this particular collection of triangles” or “idle geometric doodling” or something like that. You could label the drawing with the word triangle, but nothing about the physical facts fixes the meaning of that particular set of symbols (and the philosophical arguments from Kripke, etc, that Feser references make the case that the meanings of words are not determinate even taking the facts about the way those words are used into consideration).
And the point generalizes. There is nothing to be found about a particular string of 1s and 0s in a computer, either intrinsically or in causal and functional relationships to other material things, that can determine its meaning is “triangle” as opposed to “trilateral” (even though all triangles are trilaterals and vice-versa, the meanings of those two words are different - after all, one means having three sides and one means having three angles, and sides are different from angles). Same goes for patterns of neurons firing or any material representation whatsoever; nothing material can be determinate in the relevant sense.
That is one premise in the argument; and of course Feser argues for the other premise (that some of our thoughts are determinate in the relevant sense) as well, with the conclusion that some of our thoughts cannot be material. I.e., there is some immaterial component or property of our thoughts which just is meaning itself (so that there is no gap between meaning and representation where the above kind of skeptical argument can enter), and it is this instrinsic meaning in the immaterial component of our thoughts that makes it possible for patterns of neurons firing, strings of 1s and 0s, or a drawing on a piece of paper to derivatively mean “triangle”.