"That is enough."
Similar, but I'm not sure its exactly the same (perhaps you can tell us). "Tio" is trying to indicate the general category or set that is/will be discussed. Are we talking about people, or about vegetables, or floor waxes, or what? "Tiu" is indicating an individual, from a known, or indicated category. That specific person, that specific vegetable, that specific can of floor wax.
The category is often specified by a noun immediately after "tiu". Tiu libro estas interesa = That book is interesting; Tiu hundo apartenas al mi = That dog belongs to me. Alternatively, "tiu" can be used by itself, if the category is specified previously, or in some other way. La katoj estas ĉarmaj. Tiu estas la plej vigla. = The cats are charming. That one is the most energetic.
In a phrase like "That is enough", "that" is general and non-specific. Without other context, it specifies the general category of discourse, rather than an individual from a known category. Hence, "tio" rather than "tiu".