It’s just a statement against scientism; that everything we are able to know is limited by what scientific inquiry can tell us. I would explain it to a non-believer by giving examples of art. We can know truth from something like a painting (that an event took place). We might learn truth from a play filled with non-real characters. The point is that there are other ways to know truth besides only the application of the scientific method.
The very premise that we can only know truth through scientific inquiry is itself a philosophical statement that cannot be proven with the scientific method, making the statement itself false.
If one believes there is ONE source of all truth, then science and philosophy and art are not competitors but instead are complimentary, meaning that they should all lead to the same ultimate truth. Those who believe in scientism disbelieve that idea; they think science will lead to one thing (truth), while philosophy, art, etc. leads to something else but not truth.