In a Nutshell: “实话” refers specifically to the truth that is spoken or told between people. It's not the abstract, scientific, or philosophical “truth” (that's
真理 zhēnlǐ), but the concrete, factual information you'd share with someone. Think of it as “real talk” or “honest words.” The most common way you'll encounter it is in the phrase “说实话” (shuō shíhuà), “to speak the truth.”