iPXE is an advanced feature used for installing an operating system on a Vultr VPS. This feature is accessible in the deploy area of the Vultr Control panel, when choosing "Custom" as an operating system.
If the operating system that you're trying to install on Vultr offers an iPXE compatible image, then you can specify a URL to the iPXE file in the Vultr Control panel to launch it. Vultr doesn't provide these iPXE-compatible URLs, you would need to review whether or not the OS vendor offers one.
The iPXE URL allows you to automate commands in the iPXE boot prompt. On boot, iPXE loads the information from the URL and executes it as if it was typed into the console. See the references below.