A more thorough explanation:

A proviso is a clause in a legal document, like a contract or a law, that requires something to happen or not happen before the rest of the document can be valid. The word comes from the Latin phrase "proviso quod," which means "provided that."

For example, let's say you sign a contract to buy a car, but there's a proviso that says the sale is only valid if you get financing from a specific bank. If you can't get financing from that bank, the sale is not valid.

In a law, a proviso might create an exception to a rule. For instance, the Wilmot Proviso was a failed attempt by Congress to ban slavery in new territories acquired after the Mexican-American War. The proviso stated that slavery could not exist in those territories, except for punishment for a crime.

Another example of a proviso is the Lockean proviso, which is a condition to John Locke's theory of property. It says that people have a right to property they create from natural resources, but only if there is enough left for others to use as well.

These examples show how a proviso can be an important part of a legal document, creating conditions that must be met before the rest of the document can be valid.