What is Single Source of Truth?
A single source of truth is one agreed, authoritative location and definition for each important piece of business data, so everyone gets the same answer.
Single Source of Truth explained
It does not mean everything lives in one system. It means that for each fact (revenue, customer count) there is one place and one definition that reports draw from, rather than each team keeping its own version.
Without it, meetings turn into arguments about whose spreadsheet is right. With it, disagreements become discussions about the business instead of the data.
Example
Finance's ledger is the source for revenue; every dashboard reads revenue from it using one documented definition.