We all know about plug and play devices. They are handled by the kernel as soon as they are plugged in. These may be USB or PCI Express, or any other auto-discovered devices. But, other device types also exist, which are not hot-pluggable, and which the kernel needs to know about prior to being managed. There are I2C, UART, SPI, and other devices not wired to enumeration-capable buses.
There are real physical buses you may already know about: USB, I2S, I2C, UART, SPI, PCI, SATA, and so on. Such buses are hardware devices named controllers. Since they are a part of SoC, they can't be removed, are non-discoverable, and are also called platform devices.
People often say platform devices are on-chip devices (embedded in the SoC). In practice, this is partially true, since they are hard-wired into the chip and can't be removed. But devices connected...