To spill the beans is to reveal information that was meant to be kept secret, usually before it was intended to be shared. It often carries the implication of an accidental or careless disclosure. Someone might spill the beans about a surprise party, or spill the beans about a planned redundancy before it is officially announced.
A popular explanation involves ancient Greek democratic voting: citizens would secretly place a white bean (yes) or black bean (no) into a jar to vote. If the jar were knocked over — spilled — the outcome would be revealed before all votes were counted. While this is an appealing origin story, historians have found no direct connection between this practice and the modern English phrase.
The earliest confirmed uses in print appear in American newspapers around 1907 to 1910, where 'spilling the beans' referred to ruining plans or revealing secrets. The phrase is American in origin. The most honest assessment is that the phrase arose naturally — the image of beans (something carefully contained) being scattered uncontrollably is a natural metaphor for the chaotic release of secrets.
It is a popular theory but historians have found no evidence linking the ancient Greek voting practice directly to the English phrase. The earliest English uses appear in the early 20th century with no reference to classical antiquity.
Yes, it is informal and colloquial. In professional or formal contexts, you would say 'disclose,' 'reveal,' or 'divulge' information rather than spill the beans.