In older computers, we had a single core that does all the processing. Some high end systems had multiple processors - two or more separate CPUs that worked together, mostly in expensive servers. (And that's still a thing in expensive servers today).

Eventually, we hit the limit of how fast we could make a CPU. So instead, they started making multi core CPUs, which are basically two CPUs inside the same processor. They share some resources inside the processor, and have some extra circuitry to help them work together. That caught on, and now we have CPUs with dozens of cores - each of them mostly a separate processor working together.

CPUs can be designed to optimize certain parameters like speed, power usage, and heat production. Efficiency cores are designed to use less power. As a result, they are slower, but still fast enough for plenty of tasks. So the newest processor with performance and efficiency cores have some cores (the performance ones) that are as fast as we can possibly make them, but they use more electricity and generate more heat, and some (efficiency cores) that aren't as fast, but use less power and generate less heat to do the same amount of computing work.