Optional Features are Windows packages which can be:
Pre-installed on the image, but that feature can be enabled/disabled by the Control Panel. Configurable features are hidden, not removed from the system. Examples include .NET, Hyper-V and Windows Media Player.
Not pre-installed, but installed on demand from Windows Update servers. Examples include language support, system tools and less popular desktop apps.
Windows Servicing Stack (DISM & Windows Update) do the actual work of configuring features, or installing FOD on a live system. Only the pre-installed Features can be removed to save disk space. Otherwise, they're just a long list of optional downloads.
To save time, it is possible to download and integrate Optional Features from the FOD ISO. But most users don't need that.