For software to run on a computer, it needs to tell the computer what to do, "display this picture of a flower", "move my character to the left", "save this poem to a file".

And for a bunch of different software to all run on the same machine, they all need to use the same basic set of instructions, this is called the machine's Instruction Set.

Because the instruction set has to work for any software, these instructions don't look that readable to us, instead of "show this flower" they might be "move this bit of memory into the processor", but software builds up millions of those instructions to eventually display a flower.

Intel processors used a set of instructions that were called x86, and then when AMD made a rival processor, they made theirs use the same instruction set so that their processors would be compatible with all the software written for Intel processors (and when they needed to move from 32bit instructions to 64bit instructions, they made a new set called x64).

Meanwhile Apple computers for a long time used processors built by IBM that used IBMs PowerPC instruction set.

Now many companies are using the ARM instruction set, but ARM is still a private company you have to pay licensing fees to, so RISC-V is rising as a new, truly open source and free to use instruction set.