What's the difference between a CPU core and a CPU/GPU thread?
Please read this Wikipedia page.

A CPU core is a unit that can perform instructions.

Most CPU cores can only process one instruction at the same time.

Some CPU cores can process more (usually two) instructions at the same time. Intel calls this hyperthreading, to the operating system (Windows or Linux, etc.) and therefore  the application software (for example: Minecraft) such a CPU looks like it has two (logical) cores since it can process two instructions at the same time. At least, that's what it looks like to the software (operating system and application).

A thread is a "string of instructions" that are processed on one logical CPU core.

So a single-core but hyperthreading CPU can process two instructions at the same time so it can handle two threads at the same time as well.

To complicate things, the software switches (in time) between many threads so usually there are a lot more threads active than there are logical CPU cores present. But that's OK, if processed quickly enough, you wouldn't notice this as a user. This is because most threads (tasks) can wait a little bit for their turn to use the CPU. Only for a quick response a thread might be assigned a higher priority so that it gets preference over other threads.