I’ll try to be as non-technical as I can, though my wife tells me that even when I try to be plain I still spew “engineer talk” I can only hope this makes sense and that the other nerds out there will forgive any over-simplification.
In the old days (not that long ago!) a CPU chip contained a single processor providing all the hardware to perform any sort of calculation. The term “multi-processor” means a computer that has more than one CPU. Most “supercomputers” are multi-processor. The first-generation Mac Pros had two processors.
But as the size of the circuitry was shrunk, it became possible to fit more than one suite of processing hardware on a single CPU chip. This saves both space and power compared to a multi-processor. Some of the circuitry is shared between these suites, so each distinct, self-contained part to support calculation came to be called a “core”. Today a single CPU chip will typically have 4, 6, 8, or even more cores. An 8-core CPU has almost the same capabilities as a multi-processor with 8 separate single-core CPUs, but in a smaller more efficient package.
Every current generation Mac uses a single CPU with multiple cores.
The term “thread” refers to an ordered sequence of processing steps to be executed. It is a programming concept at heart, whereas a core is a hardware concept. Its relationship to “core” is that at the simplest level a thread is the workload that can be assigned to a single core at a time. In modern CPUs that’s an oversimplification, as engineers have invented incredibly sophisticated means to share a core across more than one thread - so called “hyper threading” - to make one core do effectively the work of two.
Threads are relevant to SketchUp because to date nobody has managed to break the heart of a 3D modeling program into threads that can run in parallel. It’s performance is therefore limited most of the time by the single-core speed of the CPU. The number of cores may matter for other purposes such as renderers, but not for SketchUp. That speed depends on both the clock speed of the CPU and the intrinsic speed of the CPU hardware. To compare two Macs you have to do some research, for example to find out whether the single-core performance of a 2.6Ghz i5 is better or worse than the single-core performance of a 2.3Ghz i7. Sometimes the difference is less or other than the raw clock speed might lead you to expect.