AIO is all in one, referring to a software, or software part that provides more complex feature-sets. It might be a blog with media management, or a solution which resizes and re-codes media, or does much more complex things.
The reason it can be important to express software as AIO, or standalone is to describe how complex they are. Someone can learn to use a single-purpose utility much more quickly as a user or administrator, because there are less things to think about. Similarly if you need to complete feature-set of some AIO software, it might be as complex to learn many smaller tools, which is part of the attractiveness.
Competent deployment of AIO utilities is naturally more complex than single-purpose utilities. It is much simpler to setup ca3db avatar cache than WordPress, because it does less. This does not mean that guides and community support may not mitigate the abstract complexity by providing installers and well written documentation.
Maintaining something smaller is generally speaking easier than maintaining something larger.
Suppliers & Vendors
One thing AIO packages tend to offer is a single entity to interact with to discuss the bundled solution. For this reason AIO solutions might be more desirable for users because they only have one place to go to if something goes wrong. An extreme example of this is SaaS software, which is often an AIO bundling of off-the-shelf technologies and code with proprietary information, support systems and additions.
- WordPress (and most CMS systems) combine storing and presenting posts, with complex plugin (including plugin for plugin), theme and child-theme hierarchies. Some of these will require yet more software, libraries and network services in order to function.