HWE stands for Hardware Enablement and is Ubuntu's term for kernel and graphics driver support.
In order to provide the latest hardware support, our Security Onion 16.04 ISO image includes the HWE stack, which is the kernel and graphics support from Ubuntu 18.04 (Linux kernel 4.15).
If you choose to use an Ubuntu image instead the Security Onion image and want the latest hardware support, you'll want to choose an image that includes the HWE stack. Some Ubuntu images (like Ubuntu Server), provide a boot menu option to enable the HWE stack.
For more information, please see https://wiki.ubuntu.com/Kernel/LTSEnablementStack.