Layman's terms as best I can.
RISC = simple instructions, executed quickly, to do something complicated you need a group of instructions. Simplicity means less transistors, means less power.
CISC = complex instructions, executed slowly, you can do complicated things with one instruction. Complexity means more transistors, means more power (consumption).
Most of the time, CPUs execute simple instructions so RISC is the way to go for performance. BUT years ago, the CPU of choice for the IBM PC was chosen by the accounting department rather than the engineering department. They went for a chip that they could get a good discount on because of existing deals and chose the Intel 8086.
This choice gave Intel lots of money to develop their chip family, making it more and more complex and crucially utilising faster and faster clock speeds. Other companies couldn't keep up, hence Apple dumping the PowerPC RISC design and jumping on the Intel bandwagon because even IBM couldn't keep up with Intels clock speeds.
The simplicity of the RISC design means you need a relatively small number of transistors to achieve 1 instruction per clock cycle. 80x86 needed many (variable number of) cycles per instruction. Intel developed the 80x86 line up to the Pentium 4 when they hit a problem. They simply couldn't get them to run faster than 3.4GHz - they were melting. So at that point they switched the architecture to the Core line which is more RISC like but internally decodes 80x86 to maintain compatibility. Still complex.
Ultimately Intel has managed to achieve 1 instruction per clock cycle (like RISC) but to achieve this they have had to have ever more complicated pipelining of instructions, meaning even more transistors, more power, more heat.
Think of a processor like a production line in a car factory, it might take 3 days to make a car, but if you have many stations all working on cars moving along a production line, after 3 days of production the cars actually come off the production line one per minute. CPUs are like this, with many instructions being worked on at the same time.
What has happened recently with the M1, is Apple has got close to Intel's clock speeds. The M1 is rumoured to be running at 3.2GHz, which is very fast for the ARM architecture. This speed means that the ARM can process its instructions at the same rate as the Intel chips but crucially using a LOT less transistors.
This has a two fold advantage for Apple, firstly their chip uses a lot less power (less transistors), generating a loss less heat. Secondly it makes their processor core very small compared with an Intel core.
The physically small core and low power means they can add more cores but crucially also means they can add more supplemental hardware to the die. They can have memory controllers and the RAM itself on chip - and can subsequently run the RAM a lot faster than your typical DDR RAM units. The whole SOC (system on a chip) thing means everything is faster.
Apple have also been able to produce their chips using smaller manufacturing methods than Intel can reliably achieve. Further adding benefits.
Intel having always been running into a dead end with CISC, sooner or later RISC was going to dominate once manufacturing and high clock speeds became mass market.
Intel have now reached that dead end. It started with mobile where battery life was the main issue. Now its the desktop where clock speed and memory performance is key. Soon, it will be the server market where 128 or 256 core ARM CPUs will trundle along feeding us our data using less electricity and crucially requiring less cooling. Thats why nVidia want to buy ARM.
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