Refers to the architecture on which your OS was installed. It's related to the CPU's instruction set, for which you can find more information here. It's comprised of two main values, x86, and x64, corresponding to 32-Bit and 64-Bit instruction sets.
Like any other section from the Environment view, it is represented as a chart, in three variations: line chart, bar chart, and pie chart.
Please note that this data refers to the OS platform and not the actual platform of your users' CPU. For example, if your user has an x64 CPU but uses an x86 OS, you'll see x86 in this chart.