The statement about CMake being a "build generator" is a common misconception.
It's not technically wrong; it just describes HOW it works, but not WHAT it does.
In the context of the question, they do the same thing: take a bunch of C/C++ files and turn them into a binary.
So, what is the real difference?
CMake is much more high-level. It's tailored to compile C++, for which you write much less build code, but can be also used for general purpose build. make has some built-in C/C++ rules as well, but they are useless at best.
CMake does a two-step build: it generates a low-level build script in ninja or make or many other generators, and then you run it. All the shell script pieces that are normally piled into Makefile are only executed at the generation stage. Thus, CMake build can be orders of magnitude faster.
The grammar of CMake is much easier to support for external tools than make's.
Once make builds an artifact, it forgets how it was built. What sources it was built from, what compiler flags? CMake tracks it, make leaves it up to you. If one of library sources was removed since the previous version of Makefile, make won't rebuild it.
Modern CMake (starting with version 3.something) works in terms of dependencies between "targets". A target is still a single output file, but it can have transitive ("public"/"interface" in CMake terms) dependencies.
These transitive dependencies can be exposed to or hidden from the dependent packages. CMake will manage directories for you. With make, you're stuck on a file-by-file and manage-directories-by-hand level.
You could code up something in make using intermediate files to cover the last two gaps, but you're on your own. make does contain a Turing complete language (even two, sometimes three counting Guile); the first two are horrible and the Guile is practically never used.
To be honest, this is what CMake and make have in common -- their languages are pretty horrible. Here's what comes to mind:
CMake has three data types: string, list, and a target with properties. make has one: string;set_property(TARGET helloworld APPEND PROPERTY INCLUDE_DIRECTORIES "${CMAKE_CURRENT_SOURCE_DIR}");