Flux is a powerful query language that allows developers to access data from various sources, including MySQL, MariaDB, and Postgres databases. It provides an extensible way to read data from these databases using SQL-like syntax. By utilizing Flux, developers can easily merge data from different sources into a single table, enabling them to analyze and gain insights from their IoT sensor deployments more effectively. The use of Flux enables companies to leverage their existing investment in traditional RDBMS systems while still taking advantage of InfluxDB's time-series database capabilities.