A universal truth is a truth which is found to be valid in all places, circumstances, and by all people. Therefore, many times it is difficult to define a truth as universal based upon the fact that not all people, in all places, under every circumstance will find it to be true.
That being said, most universal truths are generalized to be assumed to be universal. When seeking a definition for "universal truth", it is almost impossible to define. While many people have heard the phrase used, many more cannot define it solidly.
In regards to the short story "The Open Window" by Saki, one could easily generalize a universal truth for the text: Do not trust anyone but yourself.
While this is a very pessimistic outlook, unfortunately, it is true. Mr. Nuttel comes to Vera's aunt's house to find peace for his nervous condition. Instead, he is shaken so horribly one could only expect a heart-attack happening as he ran from the house.
Nuttel, if sensible enough to not trust an innocent looking 15 year old, would have had no problem at curing his nervous condition. Instead, because he trusted the young girl, he fell even further into the inability to control his nervousness.