From its website, emphasis ours:
Featured on the 13- and 15‑inch models with Touch Bar, the Apple T2 chip includes a Secure Enclave coprocessor that provides the foundation for secure boot and encrypted storage capabilities. It also consolidates many discrete controllers, including the system management controller, audio controller, and SSD controller, into one. And the Apple T2 chip brings a familiar voice to MacBook Pro — Hey Siri is always ready to open apps, find documents, play music, or answer your questions.This means 2018 MacBook Pro users can say "Hey Siri," instead of clicking on the Siri icon in the macOS menu bar or tapping on the Siri button on the Touch Bar, to activate Apple's virtual assistant on the desktop.
The functionality is enabled by the Apple T2 chip, first introduced in the iMac Pro and now included in the new MacBook Pros.
The T2 chip integrates several previously separate components, including the system management controller, image signal processor, audio controller, and SSD controller, for expanded capabilities on the new MacBook Pro.
For instance, the T2 chip's image signal processor works with the FaceTime HD camera to enable enhanced tone mapping, improved exposure control, and face detection-based auto exposure and auto white balance. On the MacBook Pro, the chip also authenticates and secures Touch ID and Apple Pay.
The T2 chip also features a Secure Enclave coprocessor for secure boot and encrypted storage capabilities.
It's unclear if the iMac Pro will eventually support "Hey Siri" too.