Boost CMake config being weird

Project ID: a302573a-79c5-49ff-b45f-10042279fe19
Code: (currently re-rewriting)

Relevant logs:
#11 0.688 -- Found Boost: /usr/lib/x86_64-linux-gnu/cmake/Boost-1.74.0/BoostConfig.cmake (found version "1.74.0")
#11 0.701 -- Could NOT find Boost: missing: system (found /usr/lib/x86_64-linux-gnu/cmake/Boost-1.74.0/BoostConfig.cmake (found suitable version "1.74.0", minimum required is "1.64"))

What does this even mean?
(note: this is my first real proper actual C++ project, and my absolute first with CMake.)
Never mind, I think I've figured it out.
I needed to install libboost-system-dev as well.