What is the meaning of truth and falsehood?

"Valid" (صحيح) and "invalid/corrupt" (فاسد) are two terms used to describe a Shar'i act. "Validity" (الصحة) means that the act conforms to the Shar'i law, and its Shar'i effects ensue. Examples include the discharge of obligation in acts of worship, and the establishment of rulings such as the permissibility of intimacy in marriage. As for "nullity" (البطلان) — which is synonymous with "corruption" (الفساد) according to the majority of scholars — it means that the act contravenes the Shar'i law, and as a result, its Shar'i effects are not valid. Thus, the obligation is not discharged in acts of worship, and the effect in transactions is not established.