A relational database. Ignition connects easily to it and can use it to store or retrieve data.
This refers to an architecture type that allows you to add and remove pieces (modules) to change the functionality, instead of installing a different piece of software for each possible configuration. We say that Ignition has a modular architecture because you can pick and choose which modules you want and ignore the rest.
A modular architecture sometimes includes the ability to add and remove functionality without stopping the software. This is referred to as a "hot swap."