This is perhaps more easily seen as:
checkIfOn := (Select[Function[{x}, x < 0.1] /@ scaledTrajectory[[iterator ;; iterator +2]], Function[{x}, x]] // Length) == 0
That is, "select, from the True or False outputs we get on testing whether each of these three scaledTrajectories is less than 0.1, those which are identically the symbol True; find the length of what we just output; see whether it's equal to 0".
It is much more neatly phrased as:
MemberQ[scaledTrajectory[[iterator ;; iterator + 2]], _?(# < 0.1 &)]