According to the Talmud (Megilla 16b), the truth to which this verse refers is the requirement for a kosher Megillas Esther scroll to be written with etched lines in the parchment.
The Maharal explains the symbolism of etched lines. First, H-Shem is straight in the sense of the letter of the law, but He is nevertheless kind. A line is also with no set beginning or end, like Torah, like Purim, and like H-Shem Himself.
Rav Yitzchak Hutner writes that etched lines refer to our hearts (parchment) on which the truth of Torah should be inscribed.